rtl-optimization/109237 - speedup bb_is_just_return

For the testcase bb_is_just_return is on top of the profile, changing
it to walk BB insns backwards puts it off the profile.  That's because
in the forward walk you have to process possibly many debug insns
but in a backward walk you very likely run into control insns first.

	PR rtl-optimization/109237
	* cfgcleanup.cc (bb_is_just_return): Walk insns backwards.