[IROutliner] Ensure instructions at end of candidate are excluded

Occasionally instructions are between the last instruction in a region,
and the following instruction as identified by the Candidate.  This
adds an extra check right before splitting a candidate that excludes the region from being split/checked for outlining to remove errors.

Tests Added:
Tranforms/IROuutliner/outlining-extra-bitcasts.ll

Reviewer: paquette, jroelofs

Differential Revision: https://reviews.llvm.org/D104142