cleanup and speed up (software_)breakpoint_inserted_here_p

Factor out common code, and use the more efficient
ALL_BP_LOCATIONS_AT_ADDR.

gdb/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (bp_location_inserted_here_p): New function,
	factored out from ...
	(breakpoint_inserted_here_p): ... here.  Use
	ALL_BP_LOCATIONS_AT_ADDR.
	(software_breakpoint_inserted_here_p): Use
	bp_location_inserted_here_p and ALL_BP_LOCATIONS_AT_ADDR.