Implicit Fallthrough on Exit Blocks

[philzook58]

We should probably stop making these implicit fallthrough.
This creates some awkwardness in regards to the layout passes and maintaining block order.
Exit blocks should perhaps have a goto to a special patch_exit label.
This may involve refactoring some specialized jmp behavior in the patcher into layout.
We may also want to consider internalizing the block ordering layout problem into a constraint problem. Clearly we want to maximize the number of fallthroughs in order to have code size savings on reduced b instructions.