[X86] Exclude invalid element types for bitcast/broadcast folding.

It looks like the fold introduced in 63f3383e can cause crashes
if the type of the bitcasted value is not a valid vector element type,
like x86_mmx.

To resolve the crash, reject invalid vector element types. The way it is
done in the patch is a bit clunky. Perhaps there's a better way to
check?

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D104792