Revert "nir/opcodes: Use fpclassify() instead of isnormal() for ldexp"

This reverts commit d7d340fb2f.

We have an isnormal() implementation available, the only problem was that
we had the wrong return type (fixed in a later patch).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88806

Acked-by: Matt Turner <mattst88@gmail.com>

src/glsl/nir/nir_opcodes.py

@@ -480,7 +480,7 @@ else
 opcode("ldexp", 0, tunsigned, [0, 0], [tfloat, tint], "", """
 dst = ldexp(src0, src1);
 /* flush denormals to zero. */
-if (fpclassify(dst) != FP_NORMAL)
+if (!isnormal(dst))
    dst = copysign(0.0f, src0);
 """)