[NVPTX] Implement NVPTXTargetLowering::getSqrtEstimate.

Summary:

This lets us lower to sqrt.approx and rsqrt.approx under more
circumstances.

* Now we emit sqrt.approx and rsqrt.approx for calls to @llvm.sqrt.f32,
  when fast-math is enabled.  Previously, we only would emit it for
  calls to @llvm.nvvm.sqrt.f.  (With this patch we no longer emit
  sqrt.approx for calls to @llvm.nvvm.sqrt.f; we rely on intcombine to
  simplify llvm.nvvm.sqrt.f into llvm.sqrt.f32.)

* Now we emit the ftz version of rsqrt.approx when ftz is enabled.
  Previously, we only emitted rsqrt.approx when ftz was disabled.

Reviewers: hfinkel

Subscribers: llvm-commits, tra, jholewinski

Differential Revision: https://reviews.llvm.org/D28508

llvm-svn: 293605