nir/range-analysis: Adjust result range of multiplication to account for flush-to...

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

Fixes piglit tests (new in piglit!110):

    - fs-underflow-fma-compare-zero.shader_test
    - fs-underflow-mul-compare-zero.shader_test

v2: Add back part of comment accidentally deleted.  Noticed by
Caio. Remove is_not_zero function as it is no longer used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111308
Fixes: fa116ce357b ("nir/range-analysis: Range tracking for ffma and flrp")
Fixes: 405de7ccb6c ("nir/range-analysis: Rudimentary value range analysis pass")
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
All Gen7+ platforms** had similar results. (Ice Lake shown)
total instructions in shared programs: 16278465 -> 16279492 (<.01%)
instructions in affected programs: 16765 -> 17792 (6.13%)
helped: 0
HURT: 23
HURT stats (abs)   min: 7 max: 275 x̄: 44.65 x̃: 8
HURT stats (rel)   min: 1.15% max: 17.51% x̄: 4.23% x̃: 1.62%
95% mean confidence interval for instructions value: 9.57 79.74
95% mean confidence interval for instructions %-change: 1.85% 6.61%
Instructions are HURT.

total cycles in shared programs: 367135159 -> 367154270 (<.01%)
cycles in affected programs: 279306 -> 298417 (6.84%)
helped: 0
HURT: 23
HURT stats (abs)   min: 13 max: 6029 x̄: 830.91 x̃: 54
HURT stats (rel)   min: 0.17% max: 45.67% x̄: 7.33% x̃: 0.49%
95% mean confidence interval for cycles value: 100.89 1560.94
95% mean confidence interval for cycles %-change: 0.94% 13.71%
Cycles are HURT.

total spills in shared programs: 8870 -> 8869 (-0.01%)
spills in affected programs: 19 -> 18 (-5.26%)
helped: 1
HURT: 0

total fills in shared programs: 21904 -> 21901 (-0.01%)
fills in affected programs: 81 -> 78 (-3.70%)
helped: 1
HURT: 0

LOST:   0
GAINED: 1

** On Broadwell, a shader was hurt for spills / fills instead of
   helped.

No changes on any earlier platforms.