[DXIL] IsSpecialFloat for 16-bit types must be emulated

[llvm-beanz]

Due to a bug in DXC, the IsSpecialFloat DXIL operations were never generated for 16-bit types. As a result drivers are not able to handle the operation, so we need to update Clang to emulate the functions using integers.

We are also tracking a change for SM 6.9 to address this issue (microsoft/hlsl-specs#521).