i965: Fix brw_swap_cmod() for LE/GE comparisons.

i965: Fix brw_swap_cmod() for LE/GE comparisons.

The idea here is to rewrite comparisons like 2 >= x with x <= 2; we want
to simply exchange arguments, not negate the condition.  If equality was
part of the original comparison, it should remain part of the swapped
version.

This is the true cause of bug #50298.  It didn't manifest itself on
Sandybridge because we embed the conditional modifier in the IF
instruction rather than emitting a CMP.  All other platforms use CMP.

It also didn't manifest itself on the master branch because commit
be5f27a84d ("glsl: Refine the loop instruction counting.") papered over
the problem.

NOTE: This is a candidate for stable release branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50298
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>