Move a morph opt to post-order (#69984)

Move a morph opt to post-order (#69984)

When changing the shape of child nodes in pre-order we must ensure we properly morph
potential promoted/demoted child nodes. Normally this happens inside
fgMorphTree which does it for child nodes, but this specific transform was not
calling fgMorphTree on the parent tree after changing the shape of child nodes.

The simplest fix is just to move the optimization to post-order which catches more cases and does not need to bother dealing with this.

Fix #61074