branching on a stride triggers validation error in bounds checking

[meganfrisella]

The program test_branch_stride1 hits a validation error in bounds checking. Interestingly, test_branch_stride2 and test_branch_stride3 are subtly different and do not trigger any error.

# validation error
def test_branch_stride1(compiler):
    @proc
    def bar(B: [i8][3,4], res: f32):
        if (stride(B, 0) == 8):
            res = 1
    @proc
    def foo(A: i8[3,4], res: f32):
        bar(A[:,:], res)

# OK
def test_branch_stride2(compiler):
    @proc
    def bar(B: [i8][3,4], res: f32):
        if (stride(B, 0) == 8):
            res = 1
    @proc
    def foo(A: i8[3,4], res: f32):
        bar(A, res)

# OK
def test_branch_stride3(compiler):
    @proc
    def bar(B: [i8][3,4], res: f32):
        a: f32
        a = 0
        if (stride(B, 0) == 8):
            a = 1
        res = a
    @proc
    def foo(A: i8[3,4], res: f32):
        bar(A[:,:], res)

Error trace for test_branch_stride1:

src/exo/API.py:51: in proc
    return Procedure(parser.result())
src/exo/API.py:173: in __init__
    CheckBounds(proc)
src/exo/boundscheck.py:567: in __init__
    body_eff = self.map_stmts(proc.body, self.rec_proc_types(proc))
src/exo/boundscheck.py:1058: in map_stmts
    eff = eff.subst(bind)
src/exo/boundscheck.py:150: in subst
    return eff_subst(env, self)
src/exo/boundscheck.py:198: in eff_subst
    [eff_subst(env, es) for es in eff.writes],
src/exo/boundscheck.py:160: in eff_subst
    pred = eff_subst(env, eff.pred) if eff.pred else None
src/exo/boundscheck.py:177: in eff_subst
    eff_subst(env, eff.lhs),
src/exo/boundscheck.py:184: in eff_subst
    return E.Stride(name, eff.dim, eff.type, eff.srcinfo)
<string>:2: in __init__
    ???
../../.venv/exo/lib/python3.12/site-packages/asdl_adt/adt.py:46: in validate
    return point_valid(val)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

obj = Effects.Stride(name=B_1, dim=0, type=LoopIR.Stride(), srcinfo=<exo.prelude.SrcInfo object at 0x1057772c0>)

    def _validator(obj):
        if obj is not None:
            if isinstance(obj, typ):
                return obj
    
            if convert:
                return typ(obj)
    
>       raise ValidationError(typ, type(obj))
E       asdl_adt.validators.ValidationError: expected: Sym, actual: Effects.Stride

../../.venv/exo/lib/python3.12/site-packages/asdl_adt/validators.py:42: ValidationError