Track liveness of memory values

[brson]

For GC we need to know the liveness of allocas. Use the LLVM intrinsics llvm.lifetime.start and llvm.lifetime.end. For testability we should track lifetimes of all allocas, add a custom pass to verify that all allocas have a lifetime and that none are used after their lifetime.

[pcwalton]

This is not necessary for version 1 of garbage collection; we can continue to zero everything out.

[pcwalton]

Not relevant with current GC plans.