[VP] Condition in vp.select|merge not a VP mask

vp.select|merge both select lanes based on a condition mask.  Unlike
other VP intrinsics the lanes are defined where the condition mask is
false. Hence, the condition mask in vp.select|mask is not a mask in the
sense of VP intrinsics.  By doing not treating the condition mask
specially, vp.select becomes the canonical VP translation of the select
instruction.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D118981