Python: Restrict the set of throwing exceptions

Technically all unary and binary expressions can throw if they dispatch to dunder implementations (such as __neq__).
But we omit these edges to obtain a more useful CFG; the cases where user code catches these exceptions are probably rare.

Author: yoff

python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll

@@ -1635,12 +1635,15 @@ private module Input implements InputSig1, InputSig2 {
    * its normal evaluation (not via an explicit `raise`/`assert`, which are
    * modeled separately).
    *
-   * The set mirrors what the legacy CFG used to flag implicitly: function
+   * `TypeError` is intentionally excluded along with exceptions thrown by dunder
+   * implementations to avoid cluttering the CFG; we expect user code to catch
+   * these only rarely.
+   *
+   * Otherwise, the set mirrors what the legacy CFG used to flag implicitly: function
    * calls (anything can raise), attribute access (`AttributeError`),
-   * subscript access (`IndexError`/`KeyError`/`TypeError`), arithmetic and
-   * comparison operators (`TypeError`/`ZeroDivisionError`), imports
+   * subscript access (`IndexError`/`KeyError`), division (`ZeroDivisionError`), imports
    * (`ImportError`/`ModuleNotFoundError`), and generator/coroutine
-   * suspension points (`await`/`yield`/`yield from`).
+   * suspension points (`await`/`yield`/`yield from` throwing `CancelledError` or `GeneratorExit`).
    *
    * Bare `Name` reads are intentionally excluded — modeling every name
    * read as `mayThrow` would explode CFG edge count for negligible
@@ -1654,11 +1657,7 @@ private module Input implements InputSig1, InputSig2 {
     or
     e instanceof Py::Subscript
     or
-    e instanceof Py::BinaryExpr
-    or
-    e instanceof Py::UnaryExpr
-    or
-    e instanceof Py::Compare
+    e instanceof Py::BinaryExpr and e.(Py::BinaryExpr).getOp() instanceof Py::Div
     or
     e instanceof Py::ImportExpr
     or