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.

Change-Id: I1ca797ff572553986cf90cab3ac0a0c1d01ad241
Reviewed-on: https://go-review.googlesource.com/10929Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>