Ranger cache dominator queries should ignore backedges.

When querying dominators for cache values, ignore back edges in
read-only mode.

	PR tree-optimization/109238
	gcc/
	* gimple-range-cache.cc (ranger_cache::resolve_dom): Ignore
	predecessors which this block dominates.

	gcc/testsuite/
	* gcc.dg/pr109238.c: New.