The reload pass inserts spill and fill instructions as needed so
instructions that operate on registers will never see a value with stack
affinity.
This is a very basic implementation, and we can't write good test cases
until we have a spilling pass.
This value affinity doesn't mean "whatever", it means that the value
does not interact with any real instructions, where "real" means
instructions that have a legal encoding.
Update the liveness verifier to check this property:
- Encoded instructions can only use real values.
- Encoded instructions define real values.
- Ghost instructions define ghost values.
An SSA value is usually biased towards a specific register class or a
stack slot, depending on the constraints of the instructions using it.
Represent this bias as an Affinity enum, and implement a merging
algorithm for updating an affinity to satisfy a new constraint.
Affinities will be computed as part of the liveness analysis. This is
not implemented yet.
