[llvm] [SLP] Fix the Vec lane overridden by the shuffle mask (PR #106341)

[Alexey Bataev via llvm-commits]

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@@ -12,7 +12,7 @@ define void @test(i8 %0, i8 %1) {
 ; CHECK-NEXT:    [[TMP5:%.*]] = insertelement <16 x i8> [[TMP4]], i8 [[TMP1]], i32 1
 ; CHECK-NEXT:    [[TMP6:%.*]] = insertelement <16 x i8> [[TMP5]], i8 0, i32 7
 ; CHECK-NEXT:    [[TMP7:%.*]] = shufflevector <8 x i8> [[TMP2]], <8 x i8> poison, <16 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP8:%.*]] = shufflevector <16 x i8> [[TMP6]], <16 x i8> [[TMP7]], <16 x i32> <i32 16, i32 17, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 poison, i32 17, i32 poison, i32 poison, i32 poison, i32 poison, i32 14, i32 15>
+; CHECK-NEXT:    [[TMP8:%.*]] = shufflevector <16 x i8> [[TMP6]], <16 x i8> [[TMP7]], <16 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 8, i32 17, i32 10, i32 11, i32 12, i32 13, i32 14, i32 15>
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alexey-bataev wrote:

I rather doubt it is correct. Here you're picking the first element from %TMP6 (which is %TMP0 or %0 in the original code), while it should be 0th element from %TMP2 (%li8 in the original code)

https://github.com/llvm/llvm-project/pull/106341