i965/fs: Allow CSE on pre-gen7 varying-index uniform loads

i965/fs: Allow CSE on pre-gen7 varying-index uniform loads

All the other expression types allowed here have inst->mlen == 0, and this
one has implied MRF writes for all of its payload, so nothing else in the
implementation should need to change.

Reduces SEND messages for loading from pull constants in kwin's Lanczos
shader from 16 to 6.  (Due to a deficiency in constant propagation, I
can't use the hack I did in the previous commit to test the performance
change)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61554
NOTE: This is a candidate for the 9.1 branch.