Gir has three optimization passes.

joinAdjacentOps
---------------
consumes         | commands without offsets
produces         | commands without offsets
unknown commands | passed through unmodified
acts on          | `moveHead`, `add`

`joinAdjacentOps` joins adjacent `moveHead`s and `add`s together. Normally
`parse` joins runs of `<>` or `+-` into one command, but if the runs are
interrupted by another character two commands get generated. `moveHead`s are
joined first, before the pass goes over the generated commands again and
joins `add`s.

transformClearLoops
-------------------
consumes         | commands without offsets
produces         | commands without offsets
unknown commands | passed through unmodified
acts on          | `loop` with one command, which is `add 1` or `add -1`

`transformClearLoops` changes loops of the form `[-]` or `[+]` into a single
`clear` command.

addOffsetProperties
-------------------
consumes         | commands without offsets
produces         | commands with offsets
unknown commands | raise UnknownIRError
acts on          | `moveHead`, `add`, `clear`, `writeByte`, `readByte`, `loop`

`addOffsetProperties` adds an `offset` property to `add`, `clear`,
`writeByte`, and `readByte`. The `offset` tells the offset at which from the
current tape head location the operations are performed. It also adds an
`isBalanced` property to loops. This property tells if execution of loop
body ends at the same cell where it began, which is useful for performing
further optimizations.