[AMDGPU] Split waterfall loop exec manipulation

[AMDGPU] Split waterfall loop exec manipulation

Split waterfall loops into multiple blocks so that exec mask
manipulation (s_and_saveexec) does not occur in the middle of
a block.

VGPR live range optimizer is updated to handle waterfall loops
spanning multiple blocks.

Reviewed By: ruiling

Differential Revision: https://reviews.llvm.org/D122200