i965: Move IR object definitions to separate header files.

i965: Move IR object definitions to separate header files.

One should be able to manipulate i965 IR without pulling the whole
FS/VEC4 visitor classes -- Optimization passes and other
transformations would ideally be visitor-agnostic.  Among other issues
this avoids a circular dependency between the header file where such
visitor-agnostic code will be defined and the main FS/VEC4 header
where both IR (layer below) and visitor (layer above) happen to be
defined.

Reviewed-by: Matt Turner <mattst88@gmail.com>