[AMDGPU] Revert scheduling to reduce spilling

[AMDGPU] Revert scheduling to reduce spilling

We can revert region schedule if new schedule decreases occupancy.
However, if we already have only one wave we would accept any new
schedule even if it blows up register pressure. Such schedule may
result in quite heavy spilling which can be avoided if we reject
this new schedule.

Differential Revision: https://reviews.llvm.org/D72181