[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Matt Arsenault via llvm-commits]

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@@ -1401,6 +1401,14 @@ def FAULTING_OP : StandardPseudoInstruction {
   let isTerminator = true;
   let isBranch = true;
 }
+def FAKE_USE : StandardPseudoInstruction {
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arsenm wrote:

I mean if there's a justification for not just re-using kill for this, it needs to be clearly documented here 

https://github.com/llvm/llvm-project/pull/86149