swift234

Add a new enum called TermKind that enables one to perform exhaustive switching over terminators.

Previously if one wanted to switch over just terminators, one would have
to use a ValueKind with a default case. This made it impossible for one
to use exhaustive switches on TermInsts and resulted in bugs (especially
when TryApply was added).

TermKind solves this problem by:

1. Having its cases defined by TERMINATOR cases in SILNodes.def.

2. Providing easy ways of mapping ValueKinds -> TermKinds. This is done
by providing a case called TermKind::Invalid and a struct
ValueKindAsTermKind which maps non-terminator ValueKinds to
TermKind::Invalid and real terminator ValueKinds to their respective
TermKind. Thus given a SILInstruction *, to switch over terminators, one
just does:

    switch (ValueKindAsTermKind(I->getKind())) {
    case TermKind::Invalid:
      ...
    case TermKind::Return:
      ...
    }

3. Providing a convenience method on TermInst called getTermKind() that
returns ValueKindAsTermKind(getKind()).

With these exhaustive switches, hopefully we can avoid such bugs in the
future.