In cases where `emitSPUpdate` produced multiple adds:
```
call foo
add 0x7FFFFFFF <--chunk size
add ...
```
`mergeSPUpdates` would incorrectly adjust the offset of the first add
producing an invalid immediate value.
This change teaches `mergeSPUpdates` to look for a subsequent add if
updating the current one would exceed the chunk size.
@phoebewang @mconst
69f202bf36