This should be NFC and is intended to allow the optimizer to delete
:enter statements (by replacing them with `nothing`), without leaving
dangling `:leave`s around. This is accomplished by having `leave` take
(a variable number of) `:enter` tokens (that are already being used by
`:pop_exception`). The semantics are that a literal `nothing` or an
SSAValue pointing to a `nothing` statement are ignored, and one
exception handler is popped for each remaining argument. The actual
value of the token is ignored, except that the verifier asserts that it
belongs to an `:enter`.

Note that we don't need to do the same for :pop_exception, because the
token generated by an `:enter` is semantically only in scope for
:pop_exception during its catch block. If we determine the `:enter` is
dead, then its catch block is guaranteed to not be executed and will be
deleted wholesale by cfg liveness.

I was considering doing something fancier where :leave is changed back
to taking an integer after optimization, but the case where the IR size
is bigger after this change (when we are `:leave`ing many handlers) is
fairly rare and likely not worth the additional complexity or time cost
to do anything special. If it does show up in size benchmarks, I'd
rather give `:leave` a special, compact encoding.