gh-126835: Remove unused docstring const

[WolframAlph]

• Issue: Move const folding to the peephole optimizer #126835

If no docstring present, extra unused constant from CFG optimization pass will reuse always preserved slot for docstring which leads to unused constant in co_consts. This breaks some unrelated tests when migrating optimizations from AST to CFG. Bug example:

cpython/Lib/test/test_compile.py

Lines 821 to 844 in 12bd15f

	@support.cpython_only
	def test_remove_unused_consts(self):
	def f():
	"docstring"
	if True:
	return "used"
	else:
	return "unused"
	self.assertEqual(f.__code__.co_consts,
	(f.__doc__, "used"))
	@support.cpython_only
	def test_remove_unused_consts_no_docstring(self):
	# the first item (None for no docstring in this case) is
	# always retained.
	def f():
	if True:
	return "used"
	else:
	return "unused"
	self.assertEqual(f.__code__.co_consts,
	(True, "used"))

First test case does not have True in co_consts but second one does even though it is not used (not used in first either) which is funny. Another example:

def f():
    return x

print(f.__code__.co_consts)  #  (None,)

None is in co_consts even though None is not used in f. It sneaks in by implicitly adding return None when compiling code unit, and it stays there because of always preserved slot for docstring.

Related comment: #126835 (comment)

[WolframAlph]

@iritkatriel let me know if you prefer having this linked to a separate issue. And also if we need to add some tests for this & news entry.

[tomasr8]

Maybe update the comment above? It's only kept when we have the docstring now