intel/compiler: Do interpolateAtOffset coordinate scaling in NIR

intel/compiler: Do interpolateAtOffset coordinate scaling in NIR

In our source languages, interpolateAtOffset() takes a floating point
offset in the range [-0.5, +0.5].  However, the hardware takes integer
valued offsets in the range [-8, 7], in units of 1/16th of a pixel.

So, we need to multiply and clamp the coordinates.  We were doing this
in the FS backend, but with the advent of IBC, I'd like to avoid doing
it twice.  This patch instead moves the lowering to NIR so we can reuse
it across both backends.

v2: Use nir_shader_instructions_pass (suggested by Eric Anholt).

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6193>