if the xlat won't yield, don't create a new interpreter

if the xlat won't yield, don't create a new interpreter

i.e. if it's "pure", and doesn't have any external side effects.

which should save a fair bit on most xlat calls.

we also check if there's an interpreter.  which is only necessary
for the test.xlat code path.  This should arguably be changed
before a v4 release