RISC-V: Support LEN_FOLD_EXTRACT_LAST auto-vectorization

RISC-V: Support LEN_FOLD_EXTRACT_LAST auto-vectorization

Consider this following case:
int __attribute__ ((noinline, noclone))
condition_reduction (int *a, int min_v)
{
  int last = 66; /* High start value.  */

  for (int i = 0; i < 4; i++)
    if (a[i] < min_v)
      last = i;

  return last;
}

--param=riscv-autovec-preference=fixed-vlmax --param=riscv-autovec-lmul=m8

condition_reduction:
	vsetvli	a4,zero,e32,m8,ta,ma
	li	a5,32
	vmv.v.x	v8,a1
	vl8re32.v	v0,0(a0)
	vid.v	v16
	vmslt.vv	v0,v0,v8
	vsetvli	zero,a5,e8,m2,ta,ma
	vcpop.m	a5,v0
	beq	a5,zero,.L2
	addi	a5,a5,-1
	vsetvli	a4,zero,e32,m8,ta,ma
	vcompress.vm	v8,v16,v0
	vslidedown.vx	v8,v8,a5
	vmv.x.s	a0,v8
	ret
.L2:
	li	a0,66
	ret

--param=riscv-autovec-preference=scalable

condition_reduction:
	csrr	a6,vlenb
	mv	a2,a0
	li	a3,32
	li	a0,66
	srli	a6,a6,2
	vsetvli	a4,zero,e32,m1,ta,ma
	vmv.v.x	v4,a1
	vid.v	v1
.L4:
	vsetvli	a5,a3,e8,mf4,tu,mu
	vsetvli	zero,a5,e32,m1,ta,ma    ----> redundant vsetvl
	vle32.v	v0,0(a2)
	vsetvli	a4,zero,e32,m1,ta,ma
	slli	a1,a5,2
	vmv.v.x	v2,a6
	vmslt.vv	v0,v0,v4
	sub	a3,a3,a5
	vmv1r.v	v3,v1
	vadd.vv	v1,v1,v2
	vsetvli	zero,a5,e8,mf4,ta,ma
	vcpop.m	a5,v0
	beq	a5,zero,.L3
	addi	a5,a5,-1
	vsetvli	a4,zero,e32,m1,ta,ma
	vcompress.vm	v2,v3,v0
	vslidedown.vx	v2,v2,a5
	vmv.x.s	a0,v2
.L3:
	sext.w	a0,a0
	add	a2,a2,a1
	bne	a3,zero,.L4
	ret

There is a redundant vsetvli instruction in VLA vectorized codes which is the VSETVL PASS issue.

vsetvl issue is not included in this patch but will be fixed soon.

gcc/ChangeLog:

	* config/riscv/autovec.md (len_fold_extract_last_<mode>): New pattern.
	* config/riscv/riscv-protos.h (enum insn_type): New enum.
	(expand_fold_extract_last): New function.
	* config/riscv/riscv-v.cc (emit_nonvlmax_slide_insn): Ditto.
	(emit_cpop_insn): Ditto.
	(emit_nonvlmax_compress_insn): Ditto.
	(expand_fold_extract_last): Ditto.
	* config/riscv/vector.md: Fix vcpop.m ratio demand.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/reduc/extract_last-1.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-10.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-11.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-12.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-13.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-14.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-2.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-3.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-4.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-5.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-6.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-7.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-8.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last-9.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-1.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-10.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-11.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-12.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-13.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-14.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-2.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-3.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-4.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-5.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-6.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-7.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-8.c: New test.
	* gcc.target/riscv/rvv/autovec/reduc/extract_last_run-9.c: New test.