Interpreter (repl): migrate types even if their size remains the same

[asterite]

This is a small fix to the repl. Hopefully we can include it for 1.6.1.

Here's the problem:

$ crystal i
icr:1:0> a = [1, 2, 3]
=> [1, 2, 3]
icr:2:0> a = "hello"
BUG: can't change type of local variable a from Array(Int32) to (Array(Int32) | String) yet (Exception)

The problem is that the repl will migrate the type of local variables if their "codegen" size changes. However, here the type changes from a reference type to a union of reference types, but that union representation's size is the same (it's just a pointer.) But the types should still be migrated (the old type must be changed to the new type.)

This PR fixes that. It now works like this:

$ bin/crystal i
Using compiled compiler at .build/crystal
icr:1:0> a = [1, 2, 3]
=> [1, 2, 3]
icr:2:0> a = "hello"
=> "hello"
icr:3:0> a
=> "hello"
icr:4:0> exit