RISC-V: Support Dynamic LMUL Cost model

This patch support dynamic LMUL cost modeling with --param=riscv-autovec-lmul=dynamic.

Consider this following case:
void
foo (int32_t *__restrict a, int32_t *__restrict b,    int32_t *__restrict c,
      int32_t *__restrict a2, int32_t *__restrict b2, int32_t *__restrict c2,
      int32_t *__restrict a3, int32_t *__restrict b3, int32_t *__restrict c3,
      int32_t *__restrict a4, int32_t *__restrict b4, int32_t *__restrict c4,
      int32_t *__restrict a5, int32_t *__restrict b5, int32_t *__restrict c5,
      int32_t *__restrict d,
      int32_t *__restrict d2,
      int32_t *__restrict d3,
      int32_t *__restrict d4,
      int32_t *__restrict d5,
      int n)
{
  for (int i = 0; i < n; i++)
    {
      a[i] = b[i] + c[i];
      b5[i] = b[i] + c[i];
      a2[i] = b2[i] + c2[i];
      a3[i] = b3[i] + c3[i];
      a4[i] = b4[i] + c4[i];
      a5[i] = a[i] + a4[i];
      d2[i] = a2[i] + c2[i];
      d3[i] = a3[i] + c3[i];
      d4[i] = a4[i] + c4[i];
      d5[i] = a[i] + a4[i];
      a[i] = a5[i] + b5[i] + a[i];

      c2[i] = a[i] + c[i];
      c3[i] = b5[i] * a5[i];
      c4[i] = a2[i] * a3[i];
      c5[i] = b5[i] * a2[i];
      c[i] = a[i] + c3[i];
      c2[i] = a[i] + c4[i];
      a5[i] = a[i] + a4[i];
      a[i] = a[i] + b5[i] + a[i] * a2[i] * a3[i] * a4[i]
      * a5[i] * c[i] * c2[i] * c3[i] * c4[i] * c5[i]
      * d[i] * d2[i] * d3[i] * d4[i] * d5[i];
    }
}

Demo: https://godbolt.org/z/x1acoMxGT

You can see it will produce register spilling if you specify LMUL >= 4

Now, with --param=riscv-autovec-lmul=dynamic.

GCC is able to pick LMUL = 2 to optimized this case.

This feature is supported by linear scan based local live ranges analysis and
compute maximum live V_REGS in specific program point of the function to determine the VF/LMUL.

Note that this patch can well handle both SLP and non-SLP loop.

Currenty approach didn't consider the later instruction scheduler which may improve the register pressure.
In this case, we are conservatively applying smaller VF/LMUL. (Not sure whether we should support live range shrink for such corner case since we don't known whether it can improve performance a lot.)

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (get_last_live_range): New function.
	(compute_nregs_for_mode): Ditto.
	(live_range_conflict_p): Ditto.
	(max_number_of_live_regs): Ditto.
	(compute_lmul): Ditto.
	(costs::prefer_new_lmul_p): Ditto.
	(costs::better_main_loop_than_p): Ditto.
	* config/riscv/riscv-vector-costs.h (struct stmt_point): New struct.
	(struct var_live_range): Ditto.
	(struct autovec_info): Ditto.
	* config/riscv/t-riscv: Update makefile for COST model.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul-mixed-1.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-1.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-2.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-3.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-4.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-5.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-6.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul1-7.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-1.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-2.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-3.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-4.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-5.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-6.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-1.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-2.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-3.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-4.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-5.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-6.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-7.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-8.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-1.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-10.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-2.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-3.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-4.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-5.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-6.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-7.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-8.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-9.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/rvv-costmodel-vect.exp: New test.

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