SH: use register names instead of numbers

[Richard Kenner]

    > This is missing a blank line.

    Between the comment and the return type?

Yes.

    There are a lot of register number definitions in the tm.h files that
    would best go into define_constants.  So there is not really an extra
    namespace pollutions, since definitions in tm.h are very visible, too.
    Exporting the defines without a prefix makes it viable to do these
    defines just once.

My concern is about the potiential for, for example, define a constant
named PLUS, which conflicts with an RTL name.  It's just too easy
to make that sort of mistake.

    If you think there is also a good application for names that are less
    visible, it is simple to add code to have an optional prefix, i.e.

Why make this user-changable?