nir/algebraic: Optimize fabs(u2f(x))

I noticed this when I tried to do frexp(float(some_unsigned)) in the
ir_unop_find_lsb lowering pass.  The code generated for frexp() uses
fabs, and this resulted in an extra instruction.  Ultimately I ended up
not using frexp.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/nir/nir_opt_algebraic.py

@@ -75,6 +75,7 @@ optimizations = [
    (('ineg', ('ineg', a)), a),
    (('fabs', ('fabs', a)), ('fabs', a)),
    (('fabs', ('fneg', a)), ('fabs', a)),
+   (('fabs', ('u2f', a)), ('u2f', a)),
    (('iabs', ('iabs', a)), ('iabs', a)),
    (('iabs', ('ineg', a)), ('iabs', a)),
    (('~fadd', a, 0.0), a),