[AMDGPU] Fix not rescheduling without clustering

Regions are sometimes skipped which should be rescheduled without memory op
clustering. RegionIdx is not incremented when iterating over regions that
are flagged to be skipped, causing the index to be incorrect.

Thanks to Vang Thao for discovering this bug!

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D85498