i965/vec4: Use NIR to do GS input remapping

i965/vec4: Use NIR to do GS input remapping

We're already doing this in the FS back-end.  This just does the same
thing in the vec4 back-end.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>