[SelectionDAG][VP] Provide expansion for VP_MERGE

This patch adds support for expanding VP_MERGE through a sequence of
vector operations producing a full-length mask setting up the elements
past EVL/pivot to be false, combining this with the original mask, and
culminating in a full-length vector select.

This expansion should work for any data type, though the only use for
RVV is for boolean vectors, which themselves rely on an expansion for
the VSELECT.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D118058