Revert a change to an integer size

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 7b0e4cadb9d46fe32a771ea06dd592e900b6cf3e..b2b922b9888876b4bbc48aaa3217edad5b2bed53 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -4544,7 +4544,10 @@ static uint32_t FinalizeHash(uint32_t hash, intptr_t hashbits) {
   hash += hash << 3;
   hash ^= hash >> 11;  // Logical shift, unsigned hash.
   hash += hash << 15;
-  hash &= (static_cast<uint32_t>(1) << hashbits) - 1;
+  // FinalizeHash gets called with values for hashbits that are bigger than 31
+  // (like kBitsPerWord - 1).  Therefore we are careful to use a type
+  // (uintptr_t) big enough to avoid undefined behavior with the left shift.
+  hash &= (static_cast<uintptr_t>(1) << hashbits) - 1;
   return (hash == 0) ? 1 : hash;
 }