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

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

[test/CodeGen/X86/fake-use-escape.ll](https://reviews.llvm.org/D41043#change-rq7Ra9xJmIVE) from the original patch triggers an assertion failure `isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!"`, file `llvm\lib\Transforms\Utils\PromoteMemoryToRegister.cpp:737`.

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