When emitting frame information, we pass all currently-live
variable names.  We clear dead register names at the end of
every basic block, but for long basic blocks we can end up
accumulating many temporary names (one per sub expression).
This ends up hugely increasing the LLVM compilation times since
we will pass hundreds of arguments to many different callsites.

Now, we remove dead variable names as soon as they die.  (Note:
user-visible names are always live since they can always be seen
by the locals() function.)  This means that we pass many fewer values
as frame arguments.

A better solution will be to have a real bytecode language which
encodes such liveness information, instead of mostly-having it and
then discarding it.