[llvm] [AMDGPU] Improve combining on v_ldexp (PR #190236)

[Petar Avramovic via llvm-commits]

https://github.com/petar-avramovic commented:

Add GlobalISel tag in the title.
I did not realize this is just a combiner issue.

Now, since this depends on assigned register banks, think is better to simplify it and only have regbankcombine (remove combines before and after legalizer).
But the problem with that is the s64 case, select will be split into two s32 selects which is harder to match.

In summary, try to make it clear that we do
s64 G_FLDEXP combine in pre-legalizer-combiner
s16 and s32 G_FLDEXP combine in regbankcombiner

https://github.com/llvm/llvm-project/pull/190236