[llvm] [LLVM] Add HasFakeUses to MachineFunction (PR #110097)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> Is it just a performance thing, so that you have a fast way to find out whether the function contains any fake uses, or does it carry more semantic information than that?

The former - there's an additional pass that will iterate over every Instruction to find and remove loads under certain conditions which are only relevant to functions with fake uses in them, which will only appear in specific developer builds (using the `-fextend-lifetimes` flag). Tracking that boolean, which can be trivially determined as part of parsing or ISel, seems like it would be preferable to either recalculating it later or adding it as a new function attribute.

https://github.com/llvm/llvm-project/pull/110097