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

[Matt Arsenault via llvm-commits]

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@@ -1099,11 +1099,46 @@ void AsmPrinter::emitFunctionEntryLabel() {
   }
 }
 
+// Recognize cases where a spilled register is reloaded solely to feed into a
+// FAKE_USE.
+static bool isLoadFeedingIntoFakeUse(const MachineInstr &MI) {
+  const MachineFunction *MF = MI.getMF();
+  const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
+
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arsenm wrote:

Should probably just pass in TII instead of doing the 4 level dance to get to it 

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