[dev.ssa] cmd/compiler/internal/ssa: Add auxint field
Add an additional int64 auxiliary field to Value.
There are two main reasons for doing this:
1) Ints in interfaces require allocation, and we store ints in Aux a lot.
2) I'd like to have both *gc.Sym and int offsets included in lots
of operations (e.g. MOVQloadidx8). It will be more efficient to
store them as separate fields instead of a pointer to a sym/int pair.
It also simplifies a bunch of code.
This is just the refactoring. I'll start using this some more in a
subsequent changelist.