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