i965: Use NIR for lowering texture swizzle

i965: Use NIR for lowering texture swizzle

Now that nir_lower_tex can do texture swizzle lowering, we can use that
instead of repeating more-or-less the same code in both backends.  This
both allows us to share code and means that things like the tg4
work-arounds are somewhat simpler because they don't have to take the
swizzle into account.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>