[NVPTX] Use approximate FP ops when unsafe-fp-math is used, and append
.ftz to instructions if the nvptx-f32ftz attribute is set to "true"

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186820 91177308-0d34-0410-b5e6-96231b3b80d8