Deprecate child types array in Tree.cpp in favor expectedChildType API

We already have an API `expectedChildType` which extracts the expected
child type from the Opcodes.enum metadata array. Rather than using a
manually maintained and error prone table, we simply extract the child
type from one source of truth metadata table.

This eliminates the need to maintain yet another table manually when
dealing with updating/adding/removing opcodes.

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>