Fix doIsEquivalentTo for IntMaskSlotTypeNode (#1664)

This fixes an internal optimization; doesn't result in a
language-observable change, so no test case is introduced.

pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java

@@ -2381,7 +2381,7 @@ public abstract class TypeNode extends PklNode {
 
     @Override
     public final boolean doIsEquivalentTo(TypeNode other) {
-      return other instanceof UIntTypeAliasTypeNode aliasTypeNode && mask == aliasTypeNode.mask;
+      return other instanceof IntMaskSlotTypeNode typeNode && mask == typeNode.mask;
     }
 
     @Override