Fix a crash on valid (if UB) bitcode that is produced for some global

constants in C++11 mode. I have no idea why it required such particular
circumstances to get here, the code seems clearly to rely upon unchecked
assumptions.

Specifically, when we decide to form an index into a struct type, we may
have gone through (at least one) zero-length array indexing round, which
would have left the offset un-adjusted, and thus not necessarily valid
for use when indexing the struct type.

This is just an canonicalization step, so the correct thing is to refuse
to canonicalize nonsensical GEPs of this form. Implemented, and test
case added.

Fixes PR12642. Pair debugged and coded with Richard Smith. =] I credit
him with most of the debugging, and preventing me from writing the wrong
code.

llvm-svn: 155466