Currently, we have mixed use of `addi` and `add(int64_t)`/`sub(int64_t)`. The former adds a 12-bit immediate while the latter
does not have a constraint on the immediate range. We should use `addi` when possible, which would help save one runtime check
about the immediate range and save the tmp register used by the latter as well. In order to make the code more readable, this
also introduces helper routines `subi`/`subiw` and adapts callsites of `addi`/`addiw` with negative immediates.