[Sema] When checking if a bitcast is appropriate between vector types, take into
consideration the num-of-elements*width-of-element width.

Disallow casts when such width is not equal between the vector types otherwise
we may end up with an invalid LLVM bitcast.

rdar://15722308.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198474 91177308-0d34-0410-b5e6-96231b3b80d8