[IfConversion] Add missing check in IfConversion/canFallThroughTo

Summary:
When trying to figure out if MBB could fallthrough to ToMBB (possibly by
falling through a bunch of other MBBs) we didn't actually check if there
was fallthrough between the last two blocks in the chain.

Reviewers: kparzysz, iteratee, MatzeB

Reviewed By: kparzysz, iteratee

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D32996

llvm-svn: 302650