We allow ghost instructions to exist if they have no side effects.
Instructions that affect control flow or that have other side effects
must be encoded.
Teach the IL verifier to enforce this. Once any instruction has an
encoding, all instructions with side effects must have an encoding.
* Replace a single-character string literal with a character literal.
* Use is_some() instead of comparing with Some(_).
* Add code-quotes around type names in comments.
* Use !...is_empty() instead of len() != 0.
* Tidy up redundant returns.
* Remove redundant .clone() calls.
* Remove unnecessary explicit lifetime parameters.
* Tidy up unnecessary '&'s.
* Add parens to make operator precedence explicit.
* Use debug_assert_eq instead of debug_assert with ==.
* Replace a &Vec argument with a &[...].
* Replace `a = a op b` with `a op= b`.
* Avoid unnecessary closures.
* Avoid .iter() and .iter_mut() for iterating over containers.
* Remove unneeded qualification.
This means that we can verify the basics with verify_context before
moving on to verifying the liveness information.
Live ranges are now verified immediately after computing them and after
register allocation is complete.
The liveness verifier will check that the live ranges are consistent
with the function. It runs as part of the register allocation pipeline
when enable_verifier is set.
The initial implementation checks the live ranges, but not the
ISA-specific constraints and affinities.
