[TUI] Rewrite register-changed decision bits.

[TUI] Rewrite register-changed decision bits.

I stumbled on the TUI's register-changed decision code before (used to
decided whether the register should be highlighted in the register
window), for it is trying to compare all the different possible states
and contents or previous/current register contents, and as such may
need updating whenever the value machinery changes to have more state.
It's just much simpler and more future proof to compare the
previous/current printable representation instead.

The bit in tui_register_format that returns early if the register has
no name gets a bit in the way of the new prototype (what to return in
that case? NULL, empty string, etc.?).  Fortunately, that check isn't
really necessary.  All the callers will have already skipped unnamed
registers.

gdb/
2013-09-05  Pedro Alves  <palves@redhat.com>

	* tui/tui-regs.c (tui_register_format): Don't look at the
	register's name here.  Return string representing register
	value instead of storing it in the data element.
	(tui_get_register): Compare register string representations
	instead of register value states and contents.