v3d: do not setup execute flags for else block in uniform control flow

v3d: do not setup execute flags for else block in uniform control flow

Either all channels executed the 'then' block, in which case all
channels will directly jump to the 'endif' block at the end of the
'then' block, or all channels execute the 'else' block (so no
execution masking is necessary).

Shader-db results:

total instructions in shared programs: 9119238 -> 9117550 (-0.02%)
instructions in affected programs: 401252 -> 399564 (-0.42%)
helped: 855
HURT: 77

total uniforms in shared programs: 3022622 -> 3022605 (<.01%)
uniforms in affected programs: 3566 -> 3549 (-0.48%)
helped: 17
HURT: 0

total max-temps in shared programs: 1327762 -> 1327774 (<.01%)
max-temps in affected programs: 619 -> 631 (1.94%)
helped: 2
HURT: 15

Reviewed-by: Eric Anholt <eric@anholt.net>