i965/nir: Inline remap_vs_attrs

i965/nir: Inline remap_vs_attrs

Now that we have nice block iterators, there's no good reason for this
to be off on it's own.  While we're here, we convert to using the NIR
const index getters/setters instead of whacking const_index values
directly.

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