[DAGCombiner] don't try to extract a fraction of a vector binop and crash (PR39893)

Because we're potentially peeking through a bitcast in this transform,
we need to use overall bitwidths rather than number of elements to
determine when it's safe to proceed.

Should fix:
https://bugs.llvm.org/show_bug.cgi?id=39893

llvm-svn: 348383