i965/fs: Improve register coalescing interference check.

i965/fs: Improve register coalescing interference check.

I always thought that the is_control_flow() -> return false check was a
bad hack, and some previous attempts to remove it have failed and have
been reverted.

The previous two patches fix some problems that caused register
coalescing to not notice some interference between registers, which the
is_control_flow() check apparently works around.

With that fixed, we can calculate interference more accurately.

total instructions in shared programs: 6261319 -> 6257917 (-0.05%)
instructions in affected programs:     346282 -> 342880 (-0.98%)
helped:                                1552

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>