Each reducer now has a virtual reducer_name function, returning its name
(the name of the class containing this reducer). This gets displayed when
using the --trace_turbo_reduction flag. Also when using this flags more
messages are displayed.

Actually when a node is replaced in-place (which is called an update
of the node), other reducers can still update it right after the
in-place replacement. When a node is really replaced (not in-place),
then we stop trying to apply reducers to it before we propagate the
reduction through the relevant nodes.

Before a message got printed only for the last reduction it went
through. So in case a node was reduced in-place several times
in a row, only the last update was printed, or none at all if after
being reduced in-place it got reduced by being replaced by another
node: only the non-in-place replacement was showed. 

Now each time an in-place reduction is applied to a node, a message
gets printed.

Bug: 
Change-Id: Id0f816fecd44c01d0253966c6decc4861be0c2fa
Reviewed-on: https://chromium-review.googlesource.com/563365Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Alexandre Talon <alexandret@google.com>
Cr-Commit-Position: refs/heads/master@{#46552}