i965/fs: Update discard jump to preserve uniform loads via sampler.

i965/fs: Update discard jump to preserve uniform loads via sampler.

Commit 17c7ead7 exposed a bug in how uniform loading happens in the
presence of discard.  It manifested itself in an application as
randomly incorrect pixels on the borders of conditional areas.

This is due to how discards jump to the end of the shader incorrectly
for some channels.  The current implementation checks each 2x2
subspan to preserve derivatives.  When uniform loading via samplers
was turned on, it uses a full execution mask, as stated in
lower_uniform_pull_constant_loads(), and only populates four channels
of the destination (see generate_uniform_pull_constant_load_gen7()).
It happens incorrectly when the first subspan has been jumped over.

The series that implemented this optimization was done before the
changes to use samplers for uniform loads.  Uniform sampler loads
use special execution masks and only populate four channels, so we
can't jump over those or corruption ensues.

This fix only jumps to the end of the shader if all relevant channels
are disabled, i.e. all 8 or 16, depending on dispatch.  This
preserves the original GLbenchmark 2.7 speedup noted in commit
beafced2.

It changes the shader assembly accordingly:

before   : (-f0.1.any4h)  halt(8)  17 2  null { align1 WE_all 1Q };
after(8) : (-f0.1.any8h)  halt(8)  17 2  null { align1 WE_all 1Q };
after(16): (-f0.1.any16h) halt(16) 17 2  null { align1 WE_all 1H };

v2: Cleaned up comments and conditional ordering.
v3: Fix typo.

Signed-off-by: Cody Northrop <cody@lunarg.com>
Reviewed-by: Mike Stroyan <mike@lunarg.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79948