For some nested pow expressions, queueing two node replacements in the
same tree traversal would mangle tree, resulting in invalid code. Fix
this by iterating over the tree until no replacements are found, and
doing one replacement each iteration.

BUG=angleproject:851

Change-Id: Ie08ab23f4bfe3d5f32726856afc61ff1f3d6c789
Reviewed-on: https://chromium-review.googlesource.com/365400Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>