TREE_READONLY in lookup_tmp_var

[Richard Kenner]

    I understood you.  I then attempted to correct your misunderstanding.
    Clearly it didn't work.

OK, I got it now.  What I was missing is that we assign to the variable
each time.  Part of that was because I didn't understand the reuse issue
clearly.  It's actually a quite clever approach because just making
sure the type is the same isn't enough.

I suggest adding some comments to clarify this: I don't think I'm the
only one who didn't "get it" the first time they looked at this.