Remove unnecessary memory statement fields.

This commit removes both WHENCE and SIZE statement fields. A new memory
structure (which will be reflected in future commits) invalidates these
fields.
Rephrase SET_NEXT()'s variables and comments.

This commit uses the unambiguous COND0 and COND1 variables for each
branch (instead of IF and ELSE), and improves the SET_NEXT() macro
comment.