RISC-V: Support in-order floating-point reduction

RISC-V: Support in-order floating-point reduction

This patch is depending on:
https://gcc.gnu.org/pipermail/gcc-patches/2023-July/624995.html

Consider this following case:
float foo (float *__restrict a, int n)
{
  float result = 1.0;
  for (int i = 0; i < n; i++)
   result += a[i];
  return result;
}

Compile with **NO** -ffast-math:

Before this patch:
<source>:4:21: missed: couldn't vectorize loop
<source>:1:7: missed: not vectorized: relevant phi not supported: result_14 = PHI <result_11(6), 1.0e+0(5)>

After this patch:
foo:
	lui	a5,%hi(.LC0)
	flw	fa0,%lo(.LC0)(a5)
	ble	a1,zero,.L4
.L3:
	vsetvli	a5,a1,e32,m1,ta,ma
	vle32.v	v1,0(a0)
	slli	a4,a5,2
	sub	a1,a1,a5
	vfmv.s.f	v2,fa0
	add	a0,a0,a4
	vfredosum.vs	v1,v1,v2     ----------> FOLD_LEFT_PLUS
	vfmv.f.s	fa0,v1
	bne	a1,zero,.L3
	ret
.L4:
	ret

gcc/ChangeLog:

	* config/riscv/autovec.md (fold_left_plus_<mode>): New pattern.
	(mask_len_fold_left_plus_<mode>): Ditto.
	* config/riscv/riscv-protos.h (enum insn_type): New enum.
	(enum reduction_type): Ditto.
	(expand_reduction): Add in-order reduction.
	* config/riscv/riscv-v.cc (emit_nonvlmax_fp_reduction_insn): New function.
	(expand_reduction): Add in-order reduction.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-1.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-2.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-3.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-4.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-5.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-6.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict-7.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict_run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/reduc_strict_run-2.c: New test.