[AMDGPU] Fix missing waitcnt issue

Ignore out of order counters when merging brackets. The fact that
there was a pending event in the old state does not guarantee that
the waitcnt was generated, so we still need to conservatively re-process
the block.

The patch fixes a correctness issue where the block was not re-processed
and the waitcnt not inserted in consequence.

Differential Revision: https://reviews.llvm.org/D117544