i965/fs: Allow copy-propagation on pull constant load values.

i965/fs: Allow copy-propagation on pull constant load values.

Given that we handle similarly-regioned GRFs registers for our copy
propagation from our UNIFORM file, there's no reason not to allow it.

The shader-db impact is negligible -- +90 instructions total, 2 shaders helped
and 7 hurt (slightly increased register pressure increased spilling), but this
is to prevent regression in other shaders when fixing copy_propagation to
reduce register pressure in the shaders that are hurt here.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>