Slightly improve logic of some operations on stap-probe.c

Slightly improve logic of some operations on stap-probe.c

This patch contains three very small improvement on the logic of some
operations we do on stap-probe.c.  They don't change what the code
does.

Pushed as obvious.

gdb/ChangeLog:
2019-05-16  Sergio Durigan Junior  <sergiodj@redhat.com>

	* stap-probe.c (stap_parse_register_operand): Make "if (*p->arg ==
	'-')" and "else if".
	(stap_parse_single_operand): Join checks for
	"gdbarch_stap_parse_special_token_p" and
	"gdbarch_stap_parse_special_token" in the same "if" statement.
	Invert check when verifying for operation on register
	displacement.