nir/lower_system_values: Refactor and use the builder.
nir/lower_system_values: Refactor and use the builder.

Now that we have a helper in the builder for system values and a helper in
core NIR to get the intrinsic opcode, there's really no point in having
things split out into a helper function.  This commit "modernizes" this
pass to use helpers better and look more like newer passes.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler