[ConstantFolding] Avoid assert when folding ptrtoint of vectorized GEP

Summary:
Got asserts in llvm::CastInst::getCastOpcode saying:
`DestBits == SrcBits && "Illegal cast to vector (wrong type or size)"' failed.

Problem seemed to be that llvm::ConstantFoldCastInstruction did
not handle ptrtoint cast of a getelementptr returning a vector
correctly. I assume such situations are quite rare, since the
GEP needs to be considered as a constant value (base pointer
being null).
The solution used here is to simply avoid the constant fold
of ptrtoint when the value is a vector. It is not supported,
and by bailing out we do not fail on assertions later on.

Reviewers: craig.topper, majnemer, davide, filcab, efriedma

Reviewed By: efriedma

Subscribers: efriedma, filcab, llvm-commits

Differential Revision: https://reviews.llvm.org/D38546

llvm-svn: 316430