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

[Matt Arsenault via llvm-commits]

arsenm wrote:

Passing through a pseudo through globalisel should be trivial; it will just need legalization handling approximately equivalent to whatever every target does for G_IMPLICIT_DEF 

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