improve VN PHI hashing

improve VN PHI hashing

This reduces the number of collisions for PHIs in the VN hashtable
by always hashing the number of predecessors and separately hashing
the block number when we never merge PHIs from different blocks.

This improves collisions seen for the PR69609 testcase dramatically.

2020-11-13  Richard Biener  <rguenther@suse.de>

	* tree-ssa-sccvn.c (vn_phi_compute_hash): Always hash the
	number of predecessors.  Hash the block number also for
	loop header PHIs.
	(expressions_equal_p): Short-cut SSA name compares, remove
	test for NULL operands.
	(vn_phi_eq): Cache number of predecessors, change inlined
	test from expressions_equal_p.