Some more predicate analysis TLC

Some more predicate analysis TLC

This limits the simple control dep also to the cd_root plus avoids
filling the lazily computed PHI def predicate in the early out path
which would leave it not simplified and normalized if it were
re-used.  It also avoids computing the use predicates when the
post-dominance early out doesn't need it.  It also syncs
predicate::use_cannot_happen with init_from_phi_def, adding the
missing PHI edge to the computed chains (the simple control dep
code already adds it).

	* gimple-predicate-analysis.cc (predicate::use_cannot_happen):
	Do simple_control_dep_chain only up to cd_root, add the PHI
	operand edge to the chains like init_from_phi_def does.
	(predicate::is_use_guarded): Speedup early out, avoid half-way
	initializing the PHI def predicate.