Implement tagToEnum# for the bytecode system.  Blargh.  We spot tail-calls
   tagToEnum# <type> arg
and emit code to push the arg, then do a bytecode test-sequence to
determine what value it is, push the relevant constructor, and merge
control flow again, at a label which does the normal tail-call
sequence: slide the constructor down to the sequel and enter it.

Blargyle, or as some would say, barferama.