[llvm] [InstCombine] Scalarize `(vec_ops (insert ?, X, Idx))` when only one element is demanded (PR #84645)

[via llvm-commits]

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@@ -547,8 +547,7 @@ define <4 x float> @insert_in_splat_variable_index(float %x, i32 %y) {
 
 define <4 x float> @insert_in_nonsplat(float %x, <4 x float> %y) {
 ; CHECK-LABEL: @insert_in_nonsplat(
-; CHECK-NEXT:    [[XV:%.*]] = insertelement <4 x float> poison, float [[X:%.*]], i64 0
-; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x float> [[XV]], <4 x float> [[Y:%.*]], <4 x i32> <i32 poison, i32 0, i32 4, i32 poison>
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goldsteinn wrote:

This is buggy, fixing.

https://github.com/llvm/llvm-project/pull/84645