More update-ssa speedup

When we do TODO_update_ssa_no_phi we already avoid computing
dominance frontiers for all blocks - it is worth to also avoid
walking all dominated blocks in the update domwalk and restrict
the walk to the SEME region with the affected blocks.  We can
do that by walking the CFG in reverse from blocks_to_update to
the common immediate dominator, marking blocks in the region
and telling the domwalk to STOP when leaving it.

For an artificial testcase with N adjacent loops with one
unswitching opportunity that takes the incremental SSA updating
off the -ftime-report radar:

 tree loop unswitching              :  11.25 (  3%)   0.09 (  5%)  11.53 (  3%)    36M (  9%)
 `- tree SSA incremental            :  35.74 (  9%)   0.07 (  4%)  36.65 (  9%)  2734k (  1%)

improves to

 tree loop unswitching              :  10.21 (  3%)   0.05 (  3%)  11.50 (  3%)    36M (  9%)
 `- tree SSA incremental            :   0.66 (  0%)   0.02 (  1%)   0.49 (  0%)  2734k (  1%)

for less localized updates the SEME region isn't likely constrained
enough so I've restricted the extra work to TODO_update_ssa_no_phi
callers.

	* tree-into-ssa.cc (rewrite_mode::REWRITE_UPDATE_REGION): New.
	(rewrite_update_dom_walker::rewrite_update_dom_walker): Update.
	(rewrite_update_dom_walker::m_in_region_flag): New.
	(rewrite_update_dom_walker::before_dom_children): If the region
	to update is marked, STOP at exits.
	(rewrite_blocks): For REWRITE_UPDATE_REGION mark the region
	to be updated.
	(dump_update_ssa): Use bitmap_empty_p.
	(update_ssa): Likewise.  Use REWRITE_UPDATE_REGION when
	TODO_update_ssa_no_phi.
	* tree-cfgcleanup.cc (cleanup_tree_cfg_noloop): Account
	pending update_ssa to the caller.

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