microsoft/compiler: do not mark sysvalues as shader-inputs

The variable here gets added to the nir_shader::system_values-list rather than the nir_shader::inputs-list, so it should have the nir_var_system_value variable mode instead of nir_var_shader_in.

Not doing this will have horrible consequences when rebasing on upstream, because all of these variables will be stored in the same list there. This means the variable-mode is the only detail left to let us know what this is.

This doesn't seem to lead to any piglit regressions, so I'm assuming this discrepancy was a mistake.