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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Rebased onto current main; the only substantive change made is that RemoveLoadsIntoFakeUses is explicitly disabled for WASM, SPIRV, and NVPTX, in exactly the same way as all other passes that require NoVRegs.

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