D3D11: Fix integer pow workaround for nested pows. 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/365400 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>