[CVP] Remove some {s|u}add.with.overflow checks.

Summary:
This adds logic to CVP to remove some overflow checks.  It uses LVI to remove
operations with at least one constant.  Specifically, this can remove many
overflow intrinsics immediately following an overflow check in the source code,
such as:

if (x < INT_MAX)
    ... x + 1 ...

Patch by Joel Galenson!

Reviewers: sanjoy, regehr

Reviewed By: sanjoy

Subscribers: fhahn, pirama, srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D39483

llvm-svn: 317911