Support folding min(poly,poly) to const

This patch adds support that tries to fold `MIN (poly, poly)` to
a constant. Consider the following C Code:

```
void foo2 (int* restrict a, int* restrict b, int n)
{
    for (int i = 0; i < 3; i += 1)
      a[i] += b[i];
}
```

Before this patch:

```
void foo2 (int * restrict a, int * restrict b, int n)
{
  vector([4,4]) int vect__7.27;
  vector([4,4]) int vect__6.26;
  vector([4,4]) int vect__4.23;
  unsigned long _32;

  <bb 2> [local count: 268435456]:
  _32 = MIN_EXPR <3, POLY_INT_CST [4, 4]>;
  vect__4.23_20 = .MASK_LEN_LOAD (a_11(D), 32B, { -1, ... }, _32, 0);
  vect__6.26_15 = .MASK_LEN_LOAD (b_12(D), 32B, { -1, ... }, _32, 0);
  vect__7.27_9 = vect__6.26_15 + vect__4.23_20;
  .MASK_LEN_STORE (a_11(D), 32B, { -1, ... }, _32, 0, vect__7.27_9); [tail call]
  return;

}
```

After this patch:

```
void foo2 (int * restrict a, int * restrict b, int n)
{
  vector([4,4]) int vect__7.27;
  vector([4,4]) int vect__6.26;
  vector([4,4]) int vect__4.23;

  <bb 2> [local count: 268435456]:
  vect__4.23_20 = .MASK_LEN_LOAD (a_11(D), 32B, { -1, ... }, 3, 0);
  vect__6.26_15 = .MASK_LEN_LOAD (b_12(D), 32B, { -1, ... }, 3, 0);
  vect__7.27_9 = vect__6.26_15 + vect__4.23_20;
  .MASK_LEN_STORE (a_11(D), 32B, { -1, ... }, 3, 0, vect__7.27_9); [tail call]
  return;

}
```

For RISC-V RVV, csrr and branch instructions can be reduced:

Before this patch:

```
foo2:
        csrr    a4,vlenb
        srli    a4,a4,2
        li      a5,3
        bleu    a5,a4,.L5
        mv      a5,a4
.L5:
        vsetvli zero,a5,e32,m1,ta,ma
        ...
```

After this patch.

```
foo2:
	vsetivli	zero,3,e32,m1,ta,ma
        ...
```

gcc/ChangeLog:

	* fold-const.cc (can_min_p): New function.
	(poly_int_binop): Try fold MIN_EXPR.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/div-1.c: Adjust.
	* gcc.target/riscv/rvv/autovec/vls/shift-3.c: Adjust.
	* gcc.target/riscv/rvv/autovec/fold-min-poly.c: New test.

0 files changed, 0 insertions, 0 deletions