nir: Recognize i2b(b2i(x)) as x.

Helps the same set of programs as the previous commit.

instructions in affected programs:     4490 -> 4346 (-3.21%)
helped:                                8

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Glenn Kennard <glenn.kennard@gmail.com>

src/glsl/nir/nir_opt_algebraic.py

@@ -188,6 +188,7 @@ optimizations = [
    (('fcsel', a, b, b), b),
 
    # Conversions
+   (('i2b', ('b2i', a)), a),
    (('f2i', ('ftrunc', a)), ('f2i', a)),
    (('f2u', ('ftrunc', a)), ('f2u', a)),