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

[Stephen Tozer via llvm-commits]

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

The only difference in the definition here is the `AsmString`, as with most of the `DBG_` classes which differ only in `AsmString` but are handled differently by MIR passes.

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