A extended value can now be changed to a third form: An alias of another
value. This is like a copy instruction, but implicit in the value table.
Value aliases are used in lieu of use-def chains which would be used to
implement replace-all-uses-with.
Added new DFG methods:
- change_to_alias() changes an existing extended value into an alias.
Primay values can't be changed, replace their definition with a copy
instruction instead.
- resolve_aliases() find the original non-alias value.
- resolve_copies() like resolve_aliases(), but also sees through
copy/spill/fill instructions.
cf1996b036