analyzer: simplify store::eval_alias

analyzer: simplify store::eval_alias

I have followup patches that add new conditions to store::eval_alias.
Rather than duplicate all conditions for symmetry, split it up and
call it on both (A, B) and (B, A).

gcc/analyzer/ChangeLog:
	* store.cc (store::eval_alias): Make const.  Split out 2nd half
	into store::eval_alias_1 and call it twice for symmetry, avoiding
	test duplication.
	(store::eval_alias_1): New function, split out from the above.
	* store.h (store::eval_alias): Make const.
	(store::eval_alias_1): New decl.