Improve VN PHI hash table handling

Improve VN PHI hash table handling

The hash function of PHIs is weak since we want to be able to CSE
them even across basic-blocks in some cases.  The following avoids
weakening the hash for cases we are never going to CSE, reducing
the number of collisions and avoiding redundant work in the
hash and equality functions.

	* tree-ssa-sccvn.cc (vn_phi_compute_hash): Key skipping
	basic block index hashing on the availability of ->cclhs.
	(vn_phi_eq): Avoid re-doing sanity checks for CSE but
	rely on ->cclhs availability.
	(vn_phi_lookup): Set ->cclhs only when we are eventually
	going to CSE the PHI.
	(vn_phi_insert): Likewise.