nir/range-analysis: Adjust result range of exp2 to account for flush-to-zero

Fixes piglit tests (new in piglit!110):

    - fs-underflow-exp2-compare-zero.shader_test

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111308


Fixes: 405de7cc ("nir/range-analysis: Rudimentary value range analysis pass")
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

Most of the shaders affected are, unsurprisingly, in Unigine Heaven.

All Gen6+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 16278207 -> 16278465 (<.01%)
instructions in affected programs: 11374 -> 11632 (2.27%)
helped: 0
HURT: 58
HURT stats (abs)   min: 2 max: 13 x̄: 4.45 x̃: 4
HURT stats (rel)   min: 0.54% max: 4.11% x̄: 2.42% x̃: 2.82%
95% mean confidence interval for instructions value: 3.77 5.13
95% mean confidence interval for instructions %-change: 2.19% 2.64%
Instructions are HURT.

total cycles in shared programs: 367134284 -> 367135159 (<.01%)
cycles in affected programs: 81207 -> 82082 (1.08%)
helped: 17
HURT: 36
helped stats (abs) min: 6 max: 356 x̄: 90.35 x̃: 6
helped stats (rel) min: 0.69% max: 21.45% x̄: 5.71% x̃: 0.78%
HURT stats (abs)   min: 4 max: 235 x̄: 66.97 x̃: 16
HURT stats (rel)   min: 0.35% max: 27.58% x̄: 5.34% x̃: 1.09%
95% mean confidence interval for cycles value: -20.36 53.38
95% mean confidence interval for cycles %-change: -1.08% 4.67%
Inconclusive result (value mean confidence interval includes 0).

No changes on any earlier platforms.

(cherry picked from commit 33ad2bab)