Speedup path discovery in predicate::use_cannot_happen

Speedup path discovery in predicate::use_cannot_happen

The following reverts a hunk from r8-5789-g11ef0b22d68cd1 that
made compute_control_dep_chain start from function entry rather
than the immediate dominator of the source block of the edge with
the undefined value on the PHI node.  Reverting at that point
does not reveal any testsuite FAIL, in particular the added
testcase still passes.  The following adjusts this to the other
function that computes predicates that hold on the PHI incoming
edges with undefined values, predicate::init_from_phi_def, which
starts at the immediate dominator of the PHI.  That's much less
likely to run into the CFG walking limit.

	* gimple-predicate-analysis.cc (predicate::use_cannot_happen):
	Start the compute_control_dep_chain walk from the immediate
	dominator of the PHI.