More update-ssa speedup

When working on a smaller region like a loop version copy the main
time spent is now dominance fast query recompute which does a full
function DFS walk.  The dominance queries within the region of
interest should be O(log n) without fast queries and we should do
on the order of O(n) of them which overall means reasonable
complexity.

For the artificial testcase I'm looking at this shaves off
considerable time again.

	* tree-into-ssa.cc (update_ssa): Do not forcefully
	re-compute dominance fast queries for TODO_update_ssa_no_phi.

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