Add an asserting ValueHandle to the block simplification code which will

fire if anything ever invalidates the assumption of a terminator
instruction being unchanged throughout the routine.

I've convinced myself that the current definition of simplification
precludes such a transformation, so I think getting some asserts
coverage that we don't violate this agreement is sufficient to make this
code safe for the foreseeable future.

Comments to the contrary or other suggestions are of course welcome. =]
The bots are now happy with this code though, so it appears the bug here
has indeed been fixed.

llvm-svn: 153401