SSA backend update

[Matthew Fluet]

> I'm inclined to ignore this problem, and move on to debugging the SSA backend
> with the native codegen, for which there are still many regression failures.
> Matthew, I'm probably gonna need some help here in understanding what native
> codegen invariants I'm violating, and how to get maximal debugging info and
> asserts on in the native codegen.

What kind of failures: compile-time (uncaught exception or asserts) or
run-time (seg-fault or wrong results)?

Quite a bit of stuff in the native codegen is under Assert.assert.  In
particular, all of the liveness checking in between phases of the
simplifier.  For seg-faults, it's a long and ardous journey with
-g -keep-pass toMOut -keep g -native-commented {1,...,5}.