[clang] [llvm] [inlineasm] Improve support for multiple inline asm constraints (e.g. "rm") (PR #197798)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

Sorry, my code review queue is totally overloaded at the moment.

I'm still hoping @nikic or @arsenm can help answer my question in https://github.com/llvm/llvm-project/pull/197798#discussion_r3375604758.

I'm still curious if we can keep support for this feature isolated to fast reg alloc?  Like this PR asserts that fast regalloc is deficient and thus all this is necessary to work around fast regalloc.  But that _still_ feels like a false premise to me.  fast regalloc is infalliable when it discovers its exhausted registers, but _I think_ we can still "pre-filter" spilling "rm" as "m".  Surely we can do something better than https://github.com/llvm/llvm-project/pull/74344 ? @MatzeB 

https://github.com/llvm/llvm-project/pull/197798