reuse_at does not work for tensors not from placeholders

[seanlatias]

The following code doesn't work, which generates a ValueError stating that member tensor cannot be found.

A = hcl.placeholder((10,10))

def foo(A):
    B = hcl.compute(A.shape, lambda y, x: A[y, x], "B")
    return hcl.compute((10, 8), lambda y, x: B[y, x] + B[y, x+1] + B[y, x+2], "C")

s = hcl.create_schedule([A], foo)
s.reuse_at(foo.B, s[foo.C], foo.C.axis[1])

[seanlatias]

Solved by #255 and the test is here.

[zhangzhiru]

I suggest we briefly describe the problem and the solution for each bug fix