[llvm] [RegAllocFast] fold foldable inline asm (PR #74344)

[Matthias Braun via llvm-commits]

MatzeB wrote:

looking around there would probably need to be some additional changes as `RegAllocFast::useVirtReg` assumes that a register is necessary for every "use" in an instruction, while for foldable ones that is actually not strictly required. But going that road seems to make more sense than to pessimistically always spill/pick "m" in the fastregalloc...

https://github.com/llvm/llvm-project/pull/74344