Revise INST_EXPAND_STKTOP so that it no longer makes use of its operand.
All the information required to do a proper expansion of the exec stack
to support expanded command invocation is already present.  The operand
doesn't provide any essential information.  By ignoring it, we eliminate
the risk that the compiler might fill in the operand with a bad stack depth
estimate value.  INST_EXPAND_STKTOP doesn't need an operand, but in order
to support loading of existing bytecodes we cannot change it now.  There's
also no need to change what the compiler tries to place in the operand,
though changing it to always be zeros would be acceptable now.