[IROutliner] Adapting to hoisted bitcasts in CodeExtractor

In commit 700d2417 the CodeExtractor
was updated so that bitcasts that have lifetime markers that beginning
outside of the region are deduplicated outside the region and are not
used as an output.  This caused a discrepancy in the IROutliner, where
in these cases there were arguments added to the aggregate function
that were not needed causing assertion errors.

The IROutliner queries the CodeExtractor twice to determine the inputs
and outputs, before and after `findAllocas` is called with the same
ValueSet for the outputs causing the duplication. This has been fixed
with a dummy ValueSet for the first call.

However, the additional bitcasts prevent us from using the same
similarity relationships that were previously defined by the
IR Similarity Analysis Pass. In these cases, we check whether the
initial version of the region being analyzed for outlining is still the
same as it was previously.  If it is not, i.e. because of the additional
bitcast instructions from the CodeExtractor, we discard the region.

Reviewers: yroux

Differential Revision: https://reviews.llvm.org/D94303