[dev.regabi] cmd/compile: avoid general traversal in deadcode
deadcode is trying to walk the statements it can find,
but it can sweep in other nodes too. Stop doing that:
only walk known statements containing statements.
Otherwise, if we put panics in expression accessors that
shouldn't be used anymore, deadcode can trip them.
deadcode would be a good candidate to rewrite using
EditChildren, but that would certainly cause toolstash
changes, since deadcode is so ad-hoc about exactly
which parts of the function it looks at. For now just
remove the general traversal and leave as is.