Fix signedness of xor const prop with zero (#2179)

Constant propagation of the Xor op folds `xor(a, SInt(0))` to
`asUInt(a)`. For comparison, Or folds to `asUInt(pad(a, W))`. This can
be a problem in the following case:

    circuit Foo :
      module Foo :
        input a: UInt<3>
        output b: UInt<4>
        b <= asUInt(xor(asSInt(a), SInt<4>(0)))

This would emit the assignment as `b = a` instead of the sign-extended
`b = {{1{a[2]}},a}`.

This requires adjusting the `pad(e, t)` function use in const prop,
which currently just inserts a `Pad` prim op with the requested output
type. However, the function advertises that it pads *to the width* of
the type `t`. Some of the folds rely on this and request the padding of
a SInt<N> to the width of a UInt<M>. But the current implementation then
then actually returns a `Pad` op with type UInt<M>, instead of the
SInt<M> that was requested.