RISC-V: Fixed failed rvv combine testcases

RISC-V: Fixed failed rvv combine testcases

This patch fixed the fellowing failed testcases on the trunk:
FAIL: gcc.target/riscv/rvv/autovec/cond/cond_widen_reduc-2.c scan-assembler-times \\tvfwredusum\\.vs\\tv[0-9]+,v[0-9]+,v[0-9]+,v0\\.t 2
...
FAIL: gcc.target/riscv/rvv/autovec/cond/cond_widen_reduc-2.c scan-assembler-times \\tvwredsumu\\.vs\\tv[0-9]+,v[0-9]+,v[0-9]+,v0\\.t 3
...

The reason for these failed testcases is the introduce of .VCOND_MASK_LEN
in midend for other bugfix and further leads to a new vcond_mask_len rtl
pattern after expand. So we need add new combine patterns handle this case.

Consider this code:

int16_t foo (int8_t *restrict a, int8_t *restrict pred)
{
  int16_t sum = 0;
  for (int i = 0; i < 16; i += 1)
    if (pred[i])
      sum += a[i];
  return sum;
}

Before this patch:
foo:
        vsetivli        zero,16,e8,m1,ta,ma
        vle8.v  v0,0(a1)
        vsetvli a5,zero,e8,m1,ta,ma
        vmsne.vi        v0,v0,0
        vsetvli zero,zero,e16,m2,ta,ma
        li      a3,0
        vmv.v.i v2,0
        vsetivli        zero,16,e16,m2,ta,ma
        vle8.v  v6,0(a0),v0.t
        vmv.s.x v1,a3
        vsetvli a5,zero,e16,m2,ta,ma
        vsext.vf2       v4,v6
        vsetivli        zero,16,e16,m2,tu,ma
        vmerge.vvm      v2,v2,v4,v0
        vsetvli a5,zero,e16,m2,ta,ma
        vredsum.vs      v2,v2,v1
        vmv.x.s a0,v2
        slliw   a0,a0,16
        sraiw   a0,a0,16
        ret

After this patch:
foo:
	vsetivli	zero,16,e16,m2,ta,ma
	li	a5,0
	vle8.v	v0,0(a1)
	vmv.s.x	v1,a5
	vsetvli	zero,zero,e8,m1,ta,ma
	vmsne.vi	v0,v0,0
	vle8.v	v2,0(a0),v0.t
	vwredsum.vs	v1,v2,v1,v0.t
	vsetvli	zero,zero,e16,m1,ta,ma
	vmv.x.s	a0,v1
	slliw	a0,a0,16
	sraiw	a0,a0,16
	ret

Combine the vsext.vf2, vmerge.vvm, and vredsum.vs instructions while
reducing the corresponding vsetvl instructions.

gcc/ChangeLog:

	* config/riscv/autovec-opt.md (*cond_len_<optab><v_double_trunc><mode>):
	New combine pattern.
	(*cond_len_<optab><v_quad_trunc><mode>): Ditto.
	(*cond_len_<optab><v_oct_trunc><mode>): Ditto.
	(*cond_len_extend<v_double_trunc><mode>): Ditto.
	(*cond_len_widen_reduc_plus_scal_<mode>): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/cond/cond_widen_reduc-1.c:
	* gcc.target/riscv/rvv/autovec/cond/cond_widen_reduc-2.c: