Optimise code for first/last child detection in fixupInsertion

The code to record the first/last child stacks was written before
I decided on using a recursive call going only one step when such
a stack was encountered.

As we only ever test the length of the stack most of the code around
calculating the stack can be thrown away, and all we need is a simple
test to see if we are the first/last child of the immediate parent.

Change-Id: Iffb03c649e166c9f89061a4d944fa07633f16aba
Merge "Optimise code for first/last child detection in fixupInsertion"