PHIOPT: Factor out some code from match_simplify_replacement

PHIOPT: Factor out some code from match_simplify_replacement

This factors out the code checking if we have an empty bb
or one statement that feeds into the phi so it can be used
when adding diamond shaped bb form to match_simplify_replacement
in the next patch. Also allows for some improvements
in the next patches too.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (empty_bb_or_one_feeding_into_p):
	New function.
	(match_simplify_replacement): Call
	empty_bb_or_one_feeding_into_p instead of doing it inline.