[SimplifyCFG] Add cut-off for InitializeUniqueCases.

The function can take a significant amount of time on some
complicated test cases, but for the currently only use of
the function we can stop the initialization much earlier
when we find out we are going to discard the result anyway
in the caller of the function.

Adding configurable cut-off points so that we avoid wasting time.
NFCI.

llvm-svn: 322248