Add --parm auto-profile-bbs

Add --parm auto-profile-bbs

This patch adds a parameter that controls whether BB profile is read from auto-profile
or we just scale guessed profile according to known counts.  This is mostly useful as
a first aid when auto-profile goes wrong.  Once we fix enough bugs I think it may be
removed but so far it is quite useful, so I decided to push it.

gcc/ChangeLog:

	* auto-profile.cc (determine_scale): Break out from ...
	(afdo_adjust_guessed_profile): ... here.
	(scale_bb_profile): New function.
	(afdo_annotate_cfg): Use it.
	* params.opt (auto-profile-bbs): New parmaeter.
	* doc/invoke.texi (auto-profile-bbs): Document.