Further reduce redundant work in the quote-minimization pass.

We were still revisiting an entire subtree after a merge/combine, even
though the grandchildren of the node in question were all unmodified.
Be more careful about recursion when we revisit nodes, since we know
that the children are all properly minimized at that point.

Change-Id: If8c48c4dbf30975d41843c61d16618bc2bdc951f
Merge "Further reduce redundant work in the quote-minimization pass."