[llvm] [VectorCombine] Add foldShuffleToIdentity (PR #88693)

[Simon Pilgrim via llvm-commits]

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@@ -192,11 +164,9 @@ define <8 x half> @splatandidentity(<8 x half> %a, <8 x half> %b) {
 
 define <8 x half> @splattwice(<8 x half> %a, <8 x half> %b) {
 ; CHECK-LABEL: @splattwice(
-; CHECK-NEXT:    [[AS:%.*]] = shufflevector <8 x half> [[A:%.*]], <8 x half> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[BS:%.*]] = shufflevector <8 x half> [[B:%.*]], <8 x half> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[AB1:%.*]] = fadd <4 x half> [[AS]], [[BS]]
-; CHECK-NEXT:    [[AB2:%.*]] = fadd <4 x half> [[AS]], [[BS]]
-; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x half> [[AB1]], <4 x half> [[AB2]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
+; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <8 x half> [[B:%.*]], <8 x half> poison, <8 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP2:%.*]] = shufflevector <8 x half> [[A:%.*]], <8 x half> poison, <8 x i32> zeroinitializer
+; CHECK-NEXT:    [[R:%.*]] = fadd <8 x half> [[TMP2]], [[TMP1]]
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RKSimon wrote:

This looks like we might need a foldBinopOfShuffles as well......

https://github.com/llvm/llvm-project/pull/88693