rtl-optimization/117922 - disable fold-mem-offsets for highly connected CFG

The PR shows fold-mem-offsets taking ages and a lot of memory computing
DU/UD chains as that requires the RD problem.  The issue is not so much
the memory required for the pruned sets but the high CFG connectivity
(and that the CFG is cyclic) which makes solving the dataflow problem
expensive.

The following adds the same limit as the one imposed by GCSE and CPROP.

	PR rtl-optimization/117922
	* fold-mem-offsets.cc (pass_fold_mem_offsets::execute):
	Do nothing for a highly connected CFG.