i965/vec4: Remove dependency of vec4_instruction on the visitor class.

i965/vec4: Remove dependency of vec4_instruction on the visitor class.

The only reason why you need a vec4_visitor to construct a
vec4_instruction is to initialize vec4_instruction::ir and
::annotation.  Instead set them from vec4_visitor::emit() just like
fs_visitor does.

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