[llvm] [AMDGPU] Legalize 64bit elements for BUILD_VECTOR on gfx942 (PR #145052)

[Matt Arsenault via llvm-commits]

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@@ -15796,6 +15814,27 @@ SITargetLowering::performExtractVectorEltCombine(SDNode *N,
     }
   }
 
+  // if PeekThoughBitcast(Vec)[MapIdx(CIdx)] == undef &&
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arsenm wrote:

This is a separate combine, that I would also expect be covered by the generic combiner 

https://github.com/llvm/llvm-project/pull/145052