[llvm] [AMDGPU] Generate waterfall for calls with SGPR(inreg) argument (PR #146997)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

This is too far removed from the existing MUBUF waterfall code, ideally they would be merged. Note we also have a much more general implementation in globalisel in regbankselect, though that currently can only deal with generic virtual registers 

This also should have some mir tests 

https://github.com/llvm/llvm-project/pull/146997