Separating label instructions?

[mhyee]

In #144, @gasche suggested separating the label instructions, to get something like

type merge_label = ML of raw_label
type branch_label = BL of raw_label

type instr =
| ...
| Merge_label of merge_label
| Branch_label of branch_label
| Goto of merge_label
| Branch of expr * branch_label * branch_label
| Osr of { label : merge_label; ... }
| ...

@o- notes:

I initially wanted to go this way. The reason I went with this version was that I wanted to keep the 1:1 mapping between our source language instruction and instruction type.

Update: also there are a number of places where you do not care about the kind of label and there it would introduce a lot of noise. I suspect you would need to extend about 20 pattern matches that currently just match | Label _.

Basically either one is fine. If you find two instructions much better I can also still change it.