nir/intrinsics: Add a second const index to load_uniform

nir/intrinsics: Add a second const index to load_uniform

In the i965 backend, we want to be able to "pull apart" the uniforms and
push some of them into the shader through a different path.  In order to do
this effectively, we need to know which variable is actually being referred
to by a given uniform load.  Previously, it was completely flattened by
nir_lower_io which made things difficult.  This adds more information to
the intrinsic to make this easier for us.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>