Update KeyPath.swift

Author: Azoy

stdlib/public/core/KeyPath.swift

@@ -1280,11 +1280,11 @@ internal struct ComputedArgumentSize {
     }
 
     set {
+      var reduced: UInt = 0
+
 #if _pointerBitWidth(_64)
-      let reduced: UInt = newValue == 16 ? 1 : 0
+      reduced = newValue == 16 ? 1 : 0
 #elseif _pointerBitWidth(_32)
-      let reduced: UInt
-
       switch newValue {
       case 8:
         reduced = 1