[llvm] [llvm][stackmaps] Include prestine registers for liveness computation. (PR #90529)

[Matthias Braun via llvm-commits]

MatzeB wrote:

nit: "pristine"

Seems I added this `FIXME` years ago in https://github.com/llvm/llvm-project/commit/24f26e6d91733d90ae24c8bbc95e052928b44bff where I did not want to change behavior given I had no actual users of stackmaps/statepoints at hand. The change does make sense to me.

I assume you some motivation for making this change now. Admittedly I am a bit confused as to why noone else noticed this before, do people have workarounds in their runtimes where they always save/restore callee-saved regs? CC folks who worked in that area while at azul @serguei-katkov @dantrushin @preames 

https://github.com/llvm/llvm-project/pull/90529