[LegalTypes] Move VSELECT node creation out of WidenVSELECTAndMask and push to 2 of the 3 callers.

One of the callers only wants the condition, but the vselect can
be simplified by getNode making it hard or impossible to retrieve
the condition.

Instead, return the condition and make the other 2 callers
responsible for creating the vselect node using the condition.
Rename the function to WidenVSELECTMask accordingly.

Differential Revision: https://reviews.llvm.org/D85468