[NVPTX] Implement llvm.fabs.f32, llvm.max.f32, etc.

Summary:
Previously these only worked via NVPTX-specific intrinsics.

This change will allow us to convert these target-specific intrinsics
into the general LLVM versions, allowing existing LLVM passes to reason
about their behavior.

It also gets us some minor codegen improvements as-is, from situations
where we canonicalize code into one of these llvm intrinsics.

Reviewers: majnemer

Subscribers: llvm-commits, jholewinski, tra

Differential Revision: https://reviews.llvm.org/D24300

llvm-svn: 281092