Bug 18228 - Fix accepting bitcasts between vectors of pointers with a
different number of elements.

Bitcasts were passing with vectors of pointers with different number of
elements since the number of elements was checking
SrcTy->getVectorNumElements() == SrcTy->getVectorNumElements() which
isn't helpful. The addrspacecast was also wrong, but that case at least
is caught by the verifier. Refactor bitcast and addrspacecast handling
in castIsValid to be more readable and fix this problem.