nir/algebraic: Eliminate a tautological compare

nir/algebraic: Eliminate a tautological compare

The value-range tracking pass that is coming is not clever enough to
know that the result of the ffma must be non-negative.  Making it that
smart will require quite a bit of work.  It might be possible to add a
special case that detects that a whole tree of fadd(fmul(fsat(a),
fneg(fsat(a))), 1.0) cannot be negative.

For cases when the comparison is used in the domain guard for a
square-root (see nir/algebraic: Simplify fsqrt domain guard), the
compare may be converted to a fmax.  This patch also handles that case.

All of the affected cases are in DiRT: Showdown.

All Gen7+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 17225365 -> 17225303 (<.01%)
instructions in affected programs: 40051 -> 39989 (-0.15%)
helped: 62
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.07% max: 0.66% x̄: 0.27% x̃: 0.26%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.31% -0.22%
Instructions are helped.

total cycles in shared programs: 360842788 -> 360842595 (<.01%)
cycles in affected programs: 1818081 -> 1817888 (-0.01%)
helped: 29
HURT: 22
helped stats (abs) min: 1 max: 206 x̄: 20.66 x̃: 14
helped stats (rel) min: <.01% max: 9.55% x̄: 0.87% x̃: 0.42%
HURT stats (abs)   min: 1 max: 108 x̄: 18.45 x̃: 7
HURT stats (rel)   min: <.01% max: 4.48% x̄: 0.56% x̃: 0.19%
95% mean confidence interval for cycles value: -14.48 6.91
95% mean confidence interval for cycles %-change: -0.71% 0.21%
Inconclusive result (value mean confidence interval includes 0).

No changes on any other Intel platform.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>