[RISCV] Use vnclip for scalable vector saturating truncation. (#88648)
Similar to #75145, but for scalable vectors.
Specifically, this patch works for the below optimization case:
## Source Code
```
define void @trunc_sat_i8i16_maxmin(ptr %x, ptr %y) {
%1 = load <vscale x 4 x i16>, ptr %x, align 16
%2 = tail call <vscale x 4 x i16> @llvm.smax.v4i16(<vscale x 4 x i16> %1, <vscale x 4 x i16> splat (i16 -128))
%3 = tail call <vscale x 4 x i16> @llvm.smin.v4i16(<vscale x 4 x i16> %2, <vscale x 4 x i16> splat (i16 127))
%4 = trunc <vscale x 4 x i16> %3 to <vscale x 4 x i8>
store <vscale x 4 x i8> %4, ptr %y, align 8
ret void
}
```
## Before this patch
[Compiler Explorer](https://godbolt.org/z/EKc9eGvo8)
```
trunc_sat_i8i16_maxmin:
vl1re16.v v8, (a0)
li a0, -128
vsetvli a2, zero, e16, m1, ta, ma
vmax.vx v8, v8, a0
li a0, 127
vmin.vx v8, v8, a0
vsetvli zero, zero, e8, mf2, ta, ma
vnsrl.wi v8, v8, 0
vse8.v v8, (a1)
ret
```
## After this patch
```
trunc_sat_i8i16_maxmin:
vsetivli zero, 4, e8, mf4, ta, ma
vle16.v v8, (a0)
vnclip.wi v8, v8, 0
vse8.v v8, (a1)
ret
```
GitOrigin-RevId: 0afc884e874043bf5207d561ab82b107e8860d41
4 files changed
