Abstract PHI and forwarder block checks in jump threader.

Abstract PHI and forwarder block checks in jump threader.

This patch abstracts out a couple common idioms in the forward
threader that I found useful while navigating the code base.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* tree-ssa-threadedge.c (has_phis_p): New.
	(forwarder_block_p): New.
	(potentially_threadable_block): Call forwarder_block_p.
	(jump_threader::thread_around_empty_blocks): Call has_phis_p.
	(jump_threader::thread_through_normal_block): Call
	forwarder_block_p.