Block stitching instance numbers are fragile

Block stitching adds instance number suffixes when instantiating blocks. New-style proc codegen might naturally clean this up, but in the interim it's annoying that instance names are fragile.

It would be better if the instance numbers were per-instantiated-block, e.g. instead of

```
inst A_0, inst B_1, inst A_2
```

you got

```
inst A_0
inst B_0
inst A_1
```