[AMDGPU] Fix function calls with flat scratch

When flat scratch is used, the stack pointer needs to be added when
writing arguments to the stack.
For buffer instructions, this is done in SelectMUBUFScratchOffen
and SelectMUBUFScratchOffset.

Move that to call argument lowering, like it is done in GlobalISel.

Differential Revision: https://reviews.llvm.org/D103166