[AMDGPU] Save WWM registers in functions

Authored by sebastian-ne on Apr 23 2021, 7:09 AM.


[AMDGPU] Save WWM registers in functions

The values of registers in inactive lanes needs to be saved during
function calls.

Save all registers used for whole wave mode, similar to how it is done
for VGPRs that are used for SGPR spilling.

Differential Revision: https://reviews.llvm.org/D99429

Reapply with fixed tests on window.