[CVP] Set NSW/NUW flags when simplifying with.overflow

[CVP] Set NSW/NUW flags when simplifying with.overflow

When CVP determines that a with.overflow intrinsic cannot overflow,
it currently inserts a simple add/sub. As we already determined that
there can be no overflow, we should add the appropriate NUW/NSW flag.

Differential Revision: https://reviews.llvm.org/D60585

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358298 91177308-0d34-0410-b5e6-96231b3b80d8