[llvm] [InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (PR #148593)

[Luke Lau via llvm-commits]

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@@ -960,15 +960,26 @@ define <3 x i31> @wide_splat3(<3 x i33> %x) {
 
 define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
 ; CHECK-LABEL: @wide_lengthening_splat(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i16> [[V:%.*]], <4 x i16> poison, <8 x i32> zeroinitializer
-; CHECK-NEXT:    [[TR:%.*]] = trunc <8 x i16> [[SHUF]] to <8 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i16> [[V:%.*]] to <4 x i8>
+; CHECK-NEXT:    [[TR:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> poison, <8 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <8 x i8> [[TR]]
 ;
   %shuf = shufflevector <4 x i16> %v, <4 x i16> %v, <8 x i32> zeroinitializer
   %tr = trunc <8 x i16> %shuf to <8 x i8>
   ret <8 x i8> %tr
 }
 
+define <4 x i8> @wide_shortening_splat(<8 x i16> %v) {
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lukel97 wrote:

Nit, add a comment above explaining that this is a negative test?

https://github.com/llvm/llvm-project/pull/148593