i965/nir: Refactor input/output lowering setup into helpers.

i965/nir: Refactor input/output lowering setup into helpers.

The code for input lowering is going to get significantly more
complicated shortly, so I wanted to pull it out.  Vertex shader inputs
are handled nearly identically regardless of vec4/scalar mode, so I
opted to not split that.

I thought about having each function actually do the lowering, but one
pass through nir_lower_io that handles all types (which weren't handled
earlier) is probably more efficient.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>