[clang] [llvm] [InstCombine] remove dead loads, such as memcpy from undef (PR #143958)

[Jameson Nash via cfe-commits]

================
@@ -3362,10 +3385,23 @@ Instruction 
*InstCombinerImpl::visitAllocSite(Instruction &MI) {
     DIB.reset(new DIBuilder(*MI.getModule(), /*AllowUnresolved=*/false));
   }
 
-  if (isAllocSiteRemovable(&MI, Users, TLI)) {
+  // Determine what getInitialValueOfAllocation would return without actually 
allocating the result.
+  bool KnowInitUndef = isa<AllocaInst>(MI);
----------------
vtjnash wrote:

Seems like a good idea, but it doesn't look like it: 
https://llvm.org/doxygen/namespacellvm.html#a108b6e33f153eda5019d322f0ac909b0

https://github.com/llvm/llvm-project/pull/143958
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