softfloat: Handle snan_bit_is_one == 0 in MIPS pickNaNMulAdd()

Only for Mips platform, and only for cases when snan_bit_is_one is 0,
correct the order of argument comparisons in pickNaNMulAdd().

For more info, see [1], page 53, section "3.5.3 NaN Propagation".

[1] "MIPS Architecture for Programmers Volume IV-j:
    The MIPS32 SIMD Architecture Module",
    Imagination Technologies LTD, Revision 1.12, February 3, 2016
Signed-off-by: NAleksandar Markovic <aleksandar.markovic@imgtec.com>
Reviewed-by: NLeon Alrae <leon.alrae@imgtec.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
[leon.alrae@imgtec.com:
 * reworded the subject of the patch
 * swapped if/else code blocks to match the commit description]
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>

fpu/softfloat-specialize.h

@@ -571,19 +571,36 @@ static int pickNaNMulAdd(flag aIsQNaN, flag aIsSNaN, flag bIsQNaN, flag bIsSNaN,
         return 3;
     }
 
-    /* Prefer sNaN over qNaN, in the a, b, c order. */
-    if (aIsSNaN) {
-        return 0;
-    } else if (bIsSNaN) {
-        return 1;
-    } else if (cIsSNaN) {
-        return 2;
-    } else if (aIsQNaN) {
-        return 0;
-    } else if (bIsQNaN) {
-        return 1;
+    if (status->snan_bit_is_one) {
+        /* Prefer sNaN over qNaN, in the a, b, c order. */
+        if (aIsSNaN) {
+            return 0;
+        } else if (bIsSNaN) {
+            return 1;
+        } else if (cIsSNaN) {
+            return 2;
+        } else if (aIsQNaN) {
+            return 0;
+        } else if (bIsQNaN) {
+            return 1;
+        } else {
+            return 2;
+        }
     } else {
-        return 2;
+        /* Prefer sNaN over qNaN, in the c, a, b order. */
+        if (cIsSNaN) {
+            return 2;
+        } else if (aIsSNaN) {
+            return 0;
+        } else if (bIsSNaN) {
+            return 1;
+        } else if (cIsQNaN) {
+            return 2;
+        } else if (aIsQNaN) {
+            return 0;
+        } else {
+            return 1;
+        }
     }
 }
 #elif defined(TARGET_PPC)