[llvm] DAG: Fix vector_shuffle -> splat fold defining undef lanes (PR #123596)

[Simon Pilgrim via llvm-commits]

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@@ -159,19 +159,18 @@ define <4 x i32> @test_urem_even_allones_eq(<4 x i32> %X) nounwind {
 ; CHECK-SSE2-LABEL: test_urem_even_allones_eq:
 ; CHECK-SSE2:       # %bb.0:
 ; CHECK-SSE2-NEXT:    pshufd {{.*#+}} xmm1 = xmm0[1,1,3,3]
-; CHECK-SSE2-NEXT:    pmuludq {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm1
-; CHECK-SSE2-NEXT:    movdqa {{.*#+}} xmm2 = [2147483648,2147483648,2147483648,2147483648]
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RKSimon wrote:

it looks like we've gone from loading a single `[2147483648,2147483648,2147483648,2147483648]` xmm to `[2147483648,u,2147483648,u]` and `[2147483648,2147483648,2147483648,2147483648]`

https://github.com/llvm/llvm-project/pull/123596