[llvm] [VP][DAGCombine] Use `simplifySelect` when combining vp.select. (PR #87342)

[Matt Arsenault via llvm-commits]

================
@@ -825,3 +825,56 @@ define <vscale x 2 x i1> @select_cond_x_cond(<vscale x 2 x i1> %x, <vscale x 2 x
   %a = call <vscale x 2 x i1> @llvm.vp.select.nxv2i1(<vscale x 2 x i1> %x, <vscale x 2 x i1> %y, <vscale x 2 x i1> %x, i32 %evl)
   ret <vscale x 2 x i1> %a
 }
+
+define <vscale x 2 x i1> @select_undef_T_F(<vscale x 2 x i1> %x, <vscale x 2 x i1> %y, i32 zeroext %evl) {
+; CHECK-LABEL: select_undef_T_F:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vmv1r.v v0, v8
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x i1> @llvm.vp.select.nxv2i1(<vscale x 2 x i1> undef, <vscale x 2 x i1> %x, <vscale x 2 x i1> %y, i32 %evl)
+  ret <vscale x 2 x i1> %a
+}
+
+define <vscale x 2 x i1> @select_undef_undef_F(<vscale x 2 x i1> %x, i32 zeroext %evl) {
+; CHECK-LABEL: select_undef_undef_F:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x i1> @llvm.vp.select.nxv2i1(<vscale x 2 x i1> undef, <vscale x 2 x i1> undef, <vscale x 2 x i1> %x, i32 %evl)
----------------
arsenm wrote:

```suggestion
  %a = call <vscale x 2 x i1> @llvm.vp.select.nxv2i1(<vscale x 2 x i1> poison, <vscale x 2 x i1> undef, <vscale x 2 x i1> %x, i32 %evl)
```

https://github.com/llvm/llvm-project/pull/87342