suspicious code in getPreStartForSignExtend (ScalarEvolution)

In getPreStartForSignExtend (ScalarEvolution.cpp) we have

  const SCEV *PreStart = SE->getAddExpr(DiffOps, SA->getNoWrapFlags());

If I understand the code correctly, we cannot unconditionally transfer
the no-wrap flags from SA to PreStart. "SCEVAddExpr(X, Step, Y) does
not signed-overflow" does not imply "SCEVAddExpr(X, Y) does not
sign-overflow" [1].

Note: I've assumed that a SCEVAddExpr(a_0, a_1, a_2 ... a_{n-1}) is
said to sign-overflow if

  sext(a_0 + a_1 + a_2 .. a_{n-1}) != sext(a_0) + sext(a_1) +
  sext(a_2) .. sext(a_{n-1})

Is this correct? This is the only semantics I could think of that
makes sense for a commutative operation.

-- Sanjoy

[1]: e.g. X = INT_SIGNED_MAX, Step = -1, Y = 1

I think your analysis here is spot on.