[DXIL][HLSL] Apply `dx.precise` to operations when `-Gis` is applied

[llvm-beanz]

We recently added the -Gis flag to the DXC driver mode, but we didn't add support for generating the dx.precise metadata on DXIL operations, which causes significant behavior differences from DXC.

You can see the observable difference between Clang and DXC here.