[llvm] Add Instruction selection support for x87 ld/st (PR #97016)

[Malay Sanghi via llvm-commits]

MalaySanghi wrote:

@RKSimon @arsenm 
I'm not sure if my previous comments got posted, I see a "pending" against it today.
fast-isel RUNS fallsback to selection dag for all the functions. It seems add/store of fp80 are not supported on fast-isel. - and I don't think there are efforts to support them.
So I think we could fast-isel test and keep this file outside GlobalIsel folder. Does that sound good?

https://github.com/llvm/llvm-project/pull/97016