CVE-2023-31146: Vyper vulnerable to OOB DynArray access when array is on both LHS and RHS of an assignment

during codegen, the length word of a dynarray is written before the data, which can result in OOB array access in the case where the dynarray is on both the lhs and rhs of an assignment. here is a minimal example producing the issue:

a:DynArray[uint256,3]
@external
def test() -> DynArray[uint256,3]:
self.a = [1,2,3]
self.a = empty(DynArray[uint256,3])
self.a = [self.a[0],self.a[1],self.a[2]]
return self.a # return [1,2,3]

and here is an example demonstrating the issue can cause data corruption across call frames:

@external
def test() -> DynArray[uint256,3]:
self.a()
return self.b() # return [1,2,3]

@internal
def a():
a: uint256 = 0
b: uint256 = 1
c: uint256 = 2
d: uint256 = 3

@internal
def b() -> DynArray[uint256,3]:
a: DynArray[uint256,3] = empty(DynArray[uint256,3])
a = [a[0],a[1],a[2]]
return a

examples involving append and pop:

@internal
def foo():
c: DynArray[uint256, 1] = []
c.append(c[0])

@internal
def foo():
c: DynArray[uint256, 1] = [1]
c[0] = c.pop()

the expected behavior in all of the above cases is to revert due to oob array access.