i965/nir/vec4: Add setup of input variables in NIR->vec4 pass

i965/nir/vec4: Add setup of input variables in NIR->vec4 pass

This implementation sets up a map of input variable offsets to source registers
that are already initialized with the corresponding register offset.

This map will then be queried when processing load_input intrinsic operations,
to obtain the correct register source from which the input data will be loaded.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>