[CVP] Generate simpler code for elided with.overflow intrinsics

Use a { iN undef, i1 false } struct as the base, and only insert
the first operand, instead of using { iN undef, i1 undef } as the
base and inserting both. This is the same as what we do in InstCombine.

Differential Revision: https://reviews.llvm.org/D67034

llvm-svn: 370573