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
R600: support f16 -> f64 conversion intrinsic.

Unfortunately, we don't seem to have a direct truncation, but the
extension can be legally split into two operations so we should
support that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213357 91177308-0d34-0410-b5e6-96231b3b80d8
R600: rename misleading fp16 test.

This test is actually going in the opposite direction to what the
filename and function name suggested.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213358 91177308-0d34-0410-b5e6-96231b3b80d8