Partial revert "Compiler: refactor and slightly optimize merging two …

…types (#12436)" (#12709)

This reverts part of commit bfc33db

The reverted part is 8d19346 (Generalize the case of a non-union type merged with a union type) from #12436

src/compiler/crystal/semantic/type_merge.cr

@@ -46,12 +46,11 @@ module Crystal
       return second if first.no_return?
       return first if second.no_return?
 
-      # Check if a non-union type is part of a union type
-      if !first.is_a?(UnionType) && second.is_a?(UnionType) && second.union_types.includes?(first)
+      if first.nil_type? && second.is_a?(UnionType) && second.union_types.includes?(first)
         return second
       end
 
-      if !second.is_a?(UnionType) && first.is_a?(UnionType) && first.union_types.includes?(second)
+      if second.nil_type? && first.is_a?(UnionType) && first.union_types.includes?(second)
         return first
       end