[NVPTX] Infer __nvvm_reflect as nounwind, readnone

[NVPTX] Infer __nvvm_reflect as nounwind, readnone

This patch simply mirrors the attributes we give to @llvm.nvvm.reflect
to the __nvvm_reflect libdevice call.  This shaves about 30% of the code
in libdevice away because of CSE opportunities.  It's also helps us
figure out that libdevice implementations of transcendental functions
don't have side-effects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265060 91177308-0d34-0410-b5e6-96231b3b80d8