When an instruction has multiple valid encodings, such as with and
without a REX prefix on x86-64, Cretonne typically picks the encoding
which gives the register allocator the most flexibility, which is
typically the longest encoding. This patch adds a pass that runs after
register allocation that picks the smallest encoding, working within the
constraints of the register allocator's choices. The result is smaller
and easier to read encodings.
In the future, we may want to merge this pass into the relaxation pass,
or possibly fold it into the final encoding step, however for now, a
discrete pass will suffice.
* Mark emit_to_memory as unsafe, and provide a safe compile_and_emit.
Mark `Context::emit_to_memory` and `MemoryCodeSink::new` as unsafe, as
`MemoryCodeSink` does not perform bounds checking when writing to
memory.
Add a `Context::compile_and_emit` function which provides a convenient
interface for doing `compile` and `emit_to_memory` in one step, and
which can also provide a safe interface, since it allocates memory of
the needed size itself.
* Mention that `MemoryCodeSink` can't guarantee that the pointer is valid.
