[AMDGPU][GlobalISel] Fix waterfall loops

- Move the `s_and exec` to its correct position before the content of
  the waterfall loop
- Use the SI_WATERFALL pseudo instruction, like for sdag, to benefit
  from optimizations
- Add support for indirect function calls

To support indirect calls, add a G_SI_CALL instruction without register
class restrictions and insert a waterfall loop when applying register
banks.

Differential Revision: https://reviews.llvm.org/D109052