softfloat: Tidy a * b + inf return

No reason to set values in 'a', when we already
have float_class_inf in 'c', and can flip that sign.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

fpu/softfloat.c

@@ -1384,9 +1384,8 @@ static FloatParts muladd_floats(FloatParts a, FloatParts b, FloatParts c,
             float_raise(float_flag_invalid, s);
             return parts_default_nan(s);
         } else {
-            a.cls = float_class_inf;
-            a.sign = c.sign ^ sign_flip;
-            return a;
+            c.sign ^= sign_flip;
+            return c;
         }
     }