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

[Juan Manuel Martinez Caamaño via llvm-commits]

https://github.com/jmmartinez commented:

The patch looks ok to me, but I'd like some other reviewer to take a look too. @arsenm @shiltian 

Just some naming concerns:

* `createWaterfall` doesn't create the waterfall code for any instruction, only for calls. So maybe we can name it `loadCallScalarOperandsFromVGPR` ?
* `loadMBUFScalarOperandsFromVGPR` creates the waterfall code for more than MBUF instructions. So maybe we can rename it `loadScalarOperandsFromVGPR`

Or switch the names to something related to `waterfall`; but be consistent across the two functions since they are closely related.

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