[dev.regabi] cmd/compile: clean up in preparation for expression Nodes
Using expression nodes restricts the set of valid SetOp operations,
because you can't SetOp across representation. Rewrite various code
to avoid crossing those as-yet-unintroduced boundaries.
This also includes choosing a single representation for any given Op.
For example, OCLOSE starts out as an OCALL, so it starts with a List
of one node and then moves that node to Left. That's no good with
real data structures, so the code picks a single canonical implementation
and prepares it during the conversion from one Op to the next.
In this case, the conversion of an OCALL to an OCLOSE now creates
a new node with Left initialized from the start. This pattern repeats.