[stdlib] Eliminate more overflow checks

This is a tiny code size improvement.

Author: lorentey

stdlib/public/core/HashTable.swift

@@ -392,7 +392,7 @@ extension _HashTable {
     }
     var wrap = false
     while true {
-      word += 1
+      word &+= 1
       if word == wordCount {
         _precondition(!wrap, "Hash table has no holes")
         wrap = true

stdlib/public/core/NativeDictionary.swift

@@ -426,7 +426,7 @@ extension _NativeDictionary { // Insertions
       let bucket = hashTable.insertNew(hashValue: hashValue)
       uncheckedInitialize(at: bucket, toKey: key, value: value)
     }
-    _storage._count += 1
+    _storage._count &+= 1
   }
 
   /// Insert a new entry into uniquely held storage.

stdlib/public/core/NativeSet.swift

@@ -360,7 +360,7 @@ extension _NativeSet { // Insertions
       let bucket = hashTable.insertNew(hashValue: hashValue)
       uncheckedInitialize(at: bucket, to: element)
     }
-    _storage._count += 1
+    _storage._count &+= 1
   }
 
   /// Insert a new element into uniquely held storage.