dillon1000/react

[be] Prune implicit breaks when flattening unused labels

A labeled block will generally end with an implicit break out of the label. 
However, if there are no _explicit_ breaks to the label, we'll end up with a 
ReactiveFunction along the lines of: 

``` 

bb1: { 

...instructions with no explicit `break bb1`... 

(implicit) break; 

} 

``` 

The `PruneUnusedLabels` pass removes such unused labels, inlining the content of 
label terminal into the surrounding block. However, we weren't pruning the 
`break`! This wasn't a problem in practice since codegen, and future passes, 
would just ignore this. But it's more correct to go and find these unnecessary 
implicit breaks and prune them, which this PR does. 

Again, this shouldn't have any impact other than producing cleaner 
ReactiveFunction data during debugging.