Internals: Dump Ast expression pattern statistics like Dfg (#7818)

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.

src/V3Dfg.cpp

@@ -959,6 +959,9 @@ void DfgVertex::unlinkDelete(DfgGraph& dfg) {
     delete this;
 }
 
+//######################################################################
+// Renders the canonical pattern S-expression for a single DfgVertex
+
 class DfgPatternString final {
     std::ostream& m_os;