i965: Avoid generating MOVs for most ir_assignment handling.

i965: Avoid generating MOVs for most ir_assignment handling.

This is a port of vec4_visitor::try_rewrite_rhs_to_dst to fs_visitor.

Not only is this technique less invasive and more robust, it also
generates better code.  Over and above the previous technique, this
reduced instruction count in shader-db by 0.28% on average and 1.4% in
the best case.

In no case did this technique result in more code than the prior method.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Eric Anholt <eric@anholt.net>