Don't try to round float80s when no precision is lost

Fixes the ceil() function. Previously, ceil(3) was 4.

emu/float80.c

@@ -50,6 +50,10 @@ static uint128_t u128_shift_right_round(uint128_t i, int shift, int sign) {
     uint64_t rest = i & ~(-1ul << (shift - 1));
 
     i >>= shift;
+    // if all the bits shifted out were zeroes, we're done
+    if (guard == 0 && rest == 0)
+        return i;
+
     switch (f80_rounding_mode) {
         case round_up:
             if (!sign)
@@ -61,12 +65,12 @@ static uint128_t u128_shift_right_round(uint128_t i, int shift, int sign) {
             break;
         case round_to_nearest:
             // if guard bit is not set, no need to round up
-            if (guard) {
-                if (rest != 0)
-                    i++; // round up
-                else if (i & 1)
-                    i++; // round to nearest even
-            }
+            if (!guard)
+                break;
+            if (rest != 0)
+                i++; // round up
+            else if (i & 1)
+                i++; // round to nearest even
             break;
     }
     return i;