[ConstraintFP] Model rounding control registers for inline asm (#92846)

We have an internal test affected by
https://github.com/llvm/llvm-project/commit/a69673615bb9f14794056470a32f70f60a52213d.
The reason is we haven't modeled rounding control registers for inline
asm under constraint FP.
Here is a reduced case:  https://godbolt.org/z/s4EsKP94e