[CGP] use IRBuilder to simplify code

This is no-functional-change-intended although there could
be intermediate variations caused by a difference in the
debug info produced by setting that from the builder's 
insertion point. 

I'm updating the IR test file associated with this code just
to show that the naming differences from using the builder
are visible.

The motivation for adding a helper function is that we are
likely to extend this code to deal with other overflow ops.

llvm-svn: 353056