regno_reg_rtx and related macros

[Richard Kenner]

I think these are *very bad* ideas.  For one thing, the convention is that
macros be upper case.  But much more importantly, it means that you can't
simply print the expression in GDB: you need to know the full name.

Why was this done in the first place?  It seems a major step backward to me.
If we do want to do this rename, I think we have to do it that way: not as
a macro.