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

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -0,0 +1,115 @@
+# Prevent the machine scheduler from moving instructions past FAKE_USE.
+# RUN: llc -run-pass machine-scheduler -o - %s | FileCheck %s
+#
+# We make sure that, beginning with the first FAKE_USE instruction,
+# no changes to the sequence of instructions are undertaken by the
+# scheduler. We don't bother to check that the order of the FAKE_USEs
+# remains the same. They should, but it is irrelevant.
----------------
OCHyams wrote:

I feels like this test has the potential to be able to be a rotten green test (i.e., what's to say that the scheduler would move things past the FAKE_USE anyway?). Possibly solvable with some creative `sed`-ing? I'm unsure.

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