RISC-V: Bugfix ICE for the vector return arg in mode switch

This patch would like to fix a ICE in mode sw for below example code.

during RTL pass: mode_sw
test.c: In function ‘vbool16_t j(vuint64m4_t)’:
test.c:15:1: internal compiler error: in create_pre_exit, at
mode-switching.cc:451
   15 | }
      | ^
0x3978f12 create_pre_exit
        __RISCV_BUILD__/../gcc/mode-switching.cc:451
0x3979e9e optimize_mode_switching
        __RISCV_BUILD__/../gcc/mode-switching.cc:849
0x397b9bc execute
        __RISCV_BUILD__/../gcc/mode-switching.cc:1324

extern size_t get_vl ();

vbool16_t
test (vuint64m4_t a)
{
  unsigned long b;
  return __riscv_vmsne_vx_u64m4_b16 (a, b, get_vl ());
}

The create_pre_exit would like to find a return value copy.  If
not, there will be a reason in assert but not available for above
sample code when vector calling convension is enabled by default.
This patch would like to override the TARGET_FUNCTION_VALUE_REGNO_P
for vector register and then we will have hard_regno_nregs for copy_num,
aka there is a return value copy.

As a side-effect of allow vector in TARGET_FUNCTION_VALUE_REGNO_P, the
TARGET_GET_RAW_RESULT_MODE will have vector mode and which is sizeless
cannot be converted to fixed_size_mode.  Thus override the hook
TARGET_GET_RAW_RESULT_MODE and return VOIDmode when the regno is-not-a
fixed_size_mode.

The below tests are passed for this patch.
* The fully riscv regression tests.
* The reproducing test in bugzilla PR114639.

	PR target/114639

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_function_value_regno_p): New func
	impl for hook TARGET_FUNCTION_VALUE_REGNO_P.
	(riscv_get_raw_result_mode): New func imple for hook
	TARGET_GET_RAW_RESULT_MODE.
	(TARGET_FUNCTION_VALUE_REGNO_P): Impl the hook.
	(TARGET_GET_RAW_RESULT_MODE): Ditto.
	* config/riscv/riscv.h (V_RETURN): New macro for vector return.
	(GP_RETURN_FIRST): New macro for the first GPR in return.
	(GP_RETURN_LAST): New macro for the last GPR in return.
	(FP_RETURN_FIRST): Diito but for FPR.
	(FP_RETURN_LAST): Ditto.
	(FUNCTION_VALUE_REGNO_P): Remove as deprecated and replace by
	TARGET_FUNCTION_VALUE_REGNO_P.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/base/pr114639-1.C: New test.
	* gcc.target/riscv/rvv/base/pr114639-1.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>

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