ARM64: Faster immediate check and fix corner cases

src/arm64/utils-arm64.cc

Index: src/arm64/utils-arm64.cc
diff --git a/src/arm64/utils-arm64.cc b/src/arm64/utils-arm64.cc
index 0cb4ea5e74fa6fa15d2c4c3b16484f9f191ac392..bde49685838e032b7d4f222c44533a2dc85d875d 100644
--- a/src/arm64/utils-arm64.cc
+++ b/src/arm64/utils-arm64.cc
@@ -78,6 +78,11 @@ int CountSetBits(uint64_t value, int width) {
 }
 
 
+uint64_t LargestPowerOf2Divisor(uint64_t value) {
+  return value & -value;
+}
+
+
 int MaskToBit(uint64_t mask) {
   ASSERT(CountSetBits(mask, 64) == 1);
   return CountTrailingZeros(mask, 64);