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

[Matt Arsenault via llvm-commits]

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+# Prevent the machine scheduler from moving instructions past FAKE_USE.
+# RUN: llc -run-pass machine-scheduler -debug-only=machine-scheduler 2>&1 -o - %s | FileCheck %s
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arsenm wrote:

This belongs in test/CodeGen/X86, not under MIR as it is not a MIR support test 

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