microsoft/spirv_to_dxil: Fix usage of IsFrontFacing input as bool
The front face intrinsic previously gets loaded as a u32 and then bitcasted to a i1, which is invalid.
This change reuses the d3d12_lower_bool_input from gallium/d3d12 for microsoft/spirv_to_dxil to correctly convert the front facing intrinsic via icmp
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