[llvm] [InstCombine] Refactor fixed and scalable binop shuffle combine. NFCI (PR #141287)

[Yingwei Zheng via llvm-commits]

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@@ -938,6 +938,28 @@ define <2 x i32> @shl_splat_constant1(<2 x i32> %x) {
   ret <2 x i32> %r
 }
 
+define <vscale x 2 x i32> @shl_splat_constant0_scalable(<vscale x 2 x i32> %x) {
+; CHECK-LABEL: @shl_splat_constant0_scalable(
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <vscale x 2 x i32> splat (i32 5), [[X:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = shufflevector <vscale x 2 x i32> [[TMP1]], <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer
+; CHECK-NEXT:    ret <vscale x 2 x i32> [[R]]
+;
+  %splat = shufflevector <vscale x 2 x i32> %x, <vscale x 2 x i32> undef, <vscale x 2 x i32> zeroinitializer
----------------
dtcxzyw wrote:

```suggestion
  %splat = shufflevector <vscale x 2 x i32> %x, <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer
```

https://github.com/llvm/llvm-project/pull/141287