[AMDGPU] Waitcnt pass: Modify the waitcnt pass to propagate info in the case...

[AMDGPU] Waitcnt pass: Modify the waitcnt pass to propagate info in the case of a single basic block loop. mergeInputScoreBrackets() does this for us; update it so that it processes the single bb's score bracket when processing the single bb's preds. It is, after all, a pred of itself, so it's score bracket is needed.

Differential Revision: https://reviews.llvm.org/D44434

llvm-svn: 327583