Split i386_stap_parse_special_token into smaller functions

This patch reorganizes code on gdb/i386-tdep.c's SystemTap SDT probe
support functions.  Before it, the code to parse special operands on x86
lived in a single, big function.  This patch creates 2 new functions
that makes the code more organized and removes a few indentation levels
(which is always good IMO).

I haven't modified anything logical in the functions, i.e., there's still
one latent bug on i386_stap_parse_special_token_triplet now.  I will soon
post a patch to fix this, and to also improve the readability of the two
new functions.

2014-01-12  Sergio Durigan Junior  <sergiodj@redhat.com>

	* i386-tdep.c (i386_stap_parse_special_token_triplet): New
	function, with code from i386_stap_parse_special_token.
	(i386_stap_parse_special_token_three_arg_disp): Likewise.
	(i386_stap_parse_special_token): Move code to the two functions
	above; simplify it.
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