Remove the expression combination counts from the default stats file,
and add a new `--dump-ast-patterns` option, which will dump new
`*_ast_patterns_*.txt` files. These contain the expression combinations
in a similar S-expression format as Dfg already produces with
`--dump-dfg-stats`. These dumps are not produced by just `--stats` as
they are fairly expensive to compute. Currently the new option will dump
at two points: just before we change to C types via widthMin usage, and
just before emit.
Geza Lore