While explicit sext instructions were handled correctly, the
implicit sext that occurs if the offset is smaller than the
pointer size blindly assumed that sext(X * Scale + Offset) is the
same as sext(X) * Scale + Offset, which is obviously not correct.
Fix this by extracting the code that handles linear expression
extension and reusing it for the implicit sext as well.
This patch adds a few test cases where currently NoAlias is returned,
but the pointers can alias if the multiply overflows while computing
a GEP index value.
dfukalov