intel/compiler: Improve fix_3src_operand()

intel/compiler: Improve fix_3src_operand()

Allow ATTR and IMM sources unconditionally (ATTR are just GRFs, IMM will
be handled by opt_combine_constants(). Both are already allowed by
opt_copy_propagation().

Also allow FIXED_GRF if the regioning is 8,8,1. Could also allow other
stride=1 regions (e.g., 4,4,1) and scalar regions but I don't think
those occur. This is sufficient to allow a pass added in a future commit
(fs_visitor::lower_linterp) to avoid emitting extra MOV instructions.

I removed the 'src.stride > 1' case because it seems wrong: 3-src
instructions on Gen6-9 are align16-only and can only do stride=1 or
stride=0. A run through Jenkins with an assert(src.stride <= 1) never
triggers, so it seems that it was dead code.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>