R600/SI: Implement sint<->fp64 conversions

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187987 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Implement fp32<->fp64 conversions

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187988 91177308-0d34-0410-b5e6-96231b3b80d8