[RISCV] Use vwadd.vx for splat vector with extension (#87249)

This patch allows `combineBinOp_VLToVWBinOp_VL` to handle patterns like
`(splat_vector (sext op))` or `(splat_vector (zext op))`. Then we can
use `vwadd.vx` and `vwadd.w` for such a case.

### Source code
```
define <vscale x 8 x i64> @vwadd_vx_splat_sext(<vscale x 8 x i32> %va, i32 %b) {
     %sb = sext i32 %b to i64
     %head = insertelement <vscale x 8 x i64> poison, i64 %sb, i32 0
     %splat = shufflevector <vscale x 8 x i64> %head, <vscale x 8 x i64> poison, <vscale x 8 x i32> zeroinitializer
     %vc = sext <vscale x 8 x i32> %va to <vscale x 8 x i64>
     %ve = add <vscale x 8 x i64> %vc, %splat
     ret <vscale x 8 x i64> %ve
}
```

### Before this patch
[Compiler Explorer](https://godbolt.org/z/sq191PsT4)
```
vwadd_vx_splat_sext:
  sext.w a0, a0
  vsetvli a1, zero, e64, m8, ta, ma
  vmv.v.x v16, a0
  vsetvli zero, zero, e32, m4, ta, ma
  vwadd.wv v16, v16, v8
  vmv8r.v v8, v16
  ret
```
### After this patch
```
vwadd_vx_splat_sext
  vsetvli a1, zero, e32, m4, ta, ma
  vwadd.vx v16, v8, a0
  vmv8r.v v8, v16
  ret
```

0 files changed, 0 insertions, 0 deletions