ValueMapper: Map uniqued nodes in post-order

The iteratitive algorithm from r265456 claimed but failed to create a
post-order traversal.  It had the same error that was fixed in the
ValueEnumerator in r266947: now, instead of pushing all operands on the
worklist at once, we pause whenever an operand gets pushed in order to
go depth-first (I know, it sounds obvious).

Sadly, I have no idea how to observe this from outside the algorithm and
so I haven't written a test.  The output should be the same; it should
just use fewer temporary nodes now.  I've added some comments that I
hope make the current logic clear enough it's unlikely to regress.

llvm-svn: 266949