[CVP] Remove unnecessary checks for empty GNWR; NFC

[CVP] Remove unnecessary checks for empty GNWR; NFC

The guaranteed no-wrap region is never empty, it always contains at
least zero, so these optimizations don't ever apply.

To make this more obviously true, replace the conversative return
in makeGNWR with an assertion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361698 91177308-0d34-0410-b5e6-96231b3b80d8