[X86] Bitcast subvector before broadcasting it.

Since r274013, we've been looking through bitcasts on broadcast inputs.
In the scalar-folding case (from a load, build_vector, or sc2vec),
the input type didn't matter, as we'd simply bitcast the resulting
scalar back.

However, when broadcasting a 128-bit-lane-aligned element, we create an
EXTRACT_SUBVECTOR.  Use proper types, by creating an extract_subvector
of the original input type.

llvm-svn: 294774