Revert "[AMDGPU] Ensure there are enough registers for wave dispatch"

This reverts 329808. That change caused a report of a failure in
test/CodeGen/MIR/AMDGPU/mir-canon-multi.mir that I didn't see. I suspect
it is an expensive-check-only error.

Change-Id: I8133f26f15e7d5ec2b09c687c12cd70e918461b0
llvm-svn: 329811