8280476: [macOS] : hotspot arm64 bug exposed by latest clang

Reviewed-by: kbarrett, adinn

src/hotspot/src/cpu/aarch64/vm/immediate_aarch64.cpp

@@ -122,9 +122,18 @@ static inline u_int32_t uimm(u_int32_t val, int hi, int lo)
 
 u_int64_t replicate(u_int64_t bits, int nbits, int count)
 {
-  u_int64_t result = 0;
-  // nbits may be 64 in which case we want mask to be -1
-  u_int64_t mask = ones(nbits);
+  assert(count > 0, "must be");
+  assert(nbits > 0, "must be");
+  assert(count * nbits <= 64, "must be");
+
+  // Special case nbits == 64 since the shift below with that nbits value
+  // would result in undefined behavior.
+  if (nbits == 64) {
+    return bits;
+  }
+
+  uint64_t result = 0;
+  uint64_t mask = ones(nbits);
   for (int i = 0; i < count ; i++) {
     result <<= nbits;
     result |= (bits & mask);