Simplify terminator when destination is known

In some circumstances, we know the destination of
a terminator. So for instance the following:
```
[mov const, reg] --> [] --[reg==const']--> [block1]
               ... --^ `--[reg!=const']--> [block2]
```
could be rewritten to:
```
[mov const, reg] --> [blocki]
... --> [] --[reg==const']--^ [block1]
          `--[reg!=const']--> [block2]
```