Make node output prefix match node structure name

Make node output prefix match node structure name

In most cases, the prefix string in a node output is the upper case of
the node structure name, e.g., MergeAppend -> MERGEAPPEND.  There were
a few exceptions that for either no apparent reason or perhaps minor
aesthetic reasons deviated from this.  In order to simplify this and
perhaps allow automatic generation without having to deal with
exception cases, make them all match.

Discussion: https://www.postgresql.org/message-id/c091e5cd-45f8-69ee-6a9b-de86912cc7e7@enterprisedb.com