Improve compute_control_dep_chain documentation

The following refactors compute_control_dep_chain slightly by
inlining is_loop_exit and factoring the check on the loop
invariant condition.  It also adds a comment as of how I
understand the code and it's current problem.

	* gimple-predicate-analysis.cc (compute_control_dep_chain):
	Inline is_loop_exit and refactor, add comment about
	loop exits.