Large scale refactoring to simplify some of the more obtuse internals of
DFG. Remove multiple redundant internal APIs, simplify representation of
variables, fix potential unsoundness in circular decomposition. No
functional change intended.
With --stats, we will print DFG pattern combinations, one per line, as
S-expressions to new stat files, together with their frequency, to aid
discovery of new peephole patterns.
Geza Lore
Wilson Snyder