R600: Handle fnearbyint

The difference from rint isn't really relevant here,
so treat them as equivalent. OpenCL doesn't have nearbyint,
so this is sort of pointless other than for completeness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211229 91177308-0d34-0410-b5e6-96231b3b80d8