Missing documentation for PRINT_REG config macro

[Richard Kenner]

There is no documentation for it and its use it not at all clear.
The only use of the macro is in print-rtl.c, when it's always called
with CODE of -1.  i386.c does indeed call it, but it would be cleaner
to call print_reg directly and reserve PRINT_REG just for RTL dumping.

What was the intent here?  Without documentation, it's hard to say.