ICU-13246 Save a few cycles by performing util64_pow multiply only when needed.

X-SVN-Rev: 40285

icu4c/source/i18n/nfrs.cpp

@@ -836,12 +836,15 @@ uint64_t util64_pow(uint32_t base, uint16_t exponent)  {
     }
     uint64_t result = 1;
     uint64_t pow = base;
-    while (exponent > 0) { 
+    while (true) {
         if ((exponent & 1) == 1) {
             result *= pow;
         }
-        pow *= pow; 
         exponent >>= 1;
+        if (exponent == 0) {
+            break;
+        }
+        pow *= pow;
     }
     return result;
 }