Remove EXT_TYPE_UNUSED in favor of IS_UNUSED

Remove EXT_TYPE_UNUSED in favor of IS_UNUSED

This means we no longer allocate an unused VAR for the retval of
instructions that support unused results.

Nearly all instructions already used the result variable only if
it was used. The only exception to this was the return value
variable for internal function call results. I've adjusted the code
to use a stack zval for the unused return case now. As we have
retval specialization now, we know that it doesn't matter.