nir/algebraic: Remove imprecise flog2 optimizations

nir/algebraic: Remove imprecise flog2 optimizations

While mathematically correct, these two optimizations result in an
expression with substantially lower precision than the original.  For any
positive finite floating-point value, log2(x) is well-defined and finite.
More precisely, it is in the range [-150, 150] so any sum of logarithms
log2(a) + log2(b) is also well-defined and finite as long as a and b are
both positive and finite.  However, if a and b are either very small or
very large, their product may get flushed to infinity or zero causing
log2(a * b) to be nowhere close to log2(a) + log2(b).

This imprecision was causing incorrect rendering in Talos Principal because
part of its HDR rendering process involves doing 8 texture operations,
clamping the result to [0, 65000], taking a dot-product with a constant,
and then taking the log2.  This is done 6 or 8 times and summed to produce
the final result which is written to a red texture.  In cases where you
have a region of the screen that is very dark, it can end up getting a
result value of -inf which is not what is intended.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96425
Cc: "11.1 11.2 12.0" <mesa-stable@lists.freedesktop.org>