[CallSiteSplitting] Do not crash when BB's terminator changes.

Change doCallSiteSplitting to iterate until we reach the terminator instruction.
tryToSplitCallSite can replace BB's terminator in case BB is a successor of
itself. Then IE will be invalidated and we also have to check the current
terminator.

Reviewers: junbuml, davidxl, davide, fhahn

Reviewed By: fhahn, junbuml

Differential Revision: https://reviews.llvm.org/D43824

llvm-svn: 326793