[NVPTX] Upgrade NVVM intrinsics in InstCombineCalls.

Summary:
There are many NVVM intrinsics that we can't entirely get rid of, but
that nonetheless often correspond to target-generic LLVM intrinsics.

For example, if flush denormals to zero (ftz) is enabled, we can convert
@llvm.nvvm.ceil.ftz.f to @llvm.ceil.f32.  On the other hand, if ftz is
disabled, we can't do this, because @llvm.ceil.f32 will be lowered to a
non-ftz PTX instruction.  In this case, we can, however, simplify the
non-ftz nvvm ceil intrinsic, @llvm.nvvm.ceil.f, to @llvm.ceil.f32.

These transformations are particularly useful because they let us
constant fold instructions that appear in libdevice, the bitcode library
that ships with CUDA and essentially functions as its libm.

Reviewers: tra

Subscribers: hfinkel, majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D28794

llvm-svn: 293244