Remove unused struct/enum

Patch by Matthias Braun!
LiveIntervalUnion is not used in RegAllocBase.

Patch by Matthias Braun!
Remove declaration of nonexistant function.

Patch by Matthias Braun!
Improve misleading comment.

Patch by Matthias Braun!
Remove unused function.

Patch by Matthias Braun!
Remove unnecessary parameter to RenumberValues.

Patch by Matthias Braun!