iris: Disable loop unrolling in GLSL IR.

iris: Disable loop unrolling in GLSL IR.

Leave it to NIR instead, like i965 does.  Thanks to Tim Arceri for
noticing that I'd left this enabled by accident.

shader-db results on Skylake:

total instructions in shared programs: 15522628 -> 15521642 (<.01%)
instructions in affected programs: 94008 -> 93022 (-1.05%)
helped: 34
HURT: 33
helped stats (abs) min: 12 max: 48 x̄: 33.82 x̃: 42
helped stats (rel) min: 0.06% max: 22.14% x̄: 9.86% x̃: 10.89%
HURT stats (abs)   min: 1 max: 16 x̄: 4.97 x̃: 3t
HURT stats (rel)   min: 0.82% max: 3.77% x̄: 1.73% x̃: 1.53%
95% mean confidence interval for instructions value: -20.08 -9.35
95% mean confidence interval for instructions %-change: -5.95% -2.36%
Instructions are helped.

total cycles in shared programs: 367105221 -> 367074230 (<.01%)
cycles in affected programs: 10017660 -> 9986669 (-0.31%)
helped: 266
HURT: 184
helped stats (abs) min: 1 max: 9556 x̄: 151.35 x̃: 12
helped stats (rel) min: 0.08% max: 59.91% x̄: 4.66% x̃: 1.67%
HURT stats (abs)   min: 1 max: 1716 x̄: 50.37 x̃: 6
HURT stats (rel)   min: <.01% max: 24.40% x̄: 2.42% x̃: 0.85%
95% mean confidence interval for cycles value: -133.90 -3.84
95% mean confidence interval for cycles %-change: -2.44% -1.10%
Cycles are helped.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>