NVPTX: support direct f16 <-> f64 conversions via intrinsics.

Clang may well start emitting these soon, and while it may not be
directly relevant for OpenCL or GLSL, the instructions were just
sitting there waiting to be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213356 91177308-0d34-0410-b5e6-96231b3b80d8