[LegalizeTypes][VP] Add widening support for binary VP ops

This patch adds the beginnings of more thorough support in the
legalizers for vector-predicated (VP) operations.

The first step is the ability to widen illegal vectors. The more
complicated scenario in which the result/operands need widening but the
mask doesn't has not been handled here. That would require a lot of code
without an in-tree target on which to test it.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D107904