[BasicAA] Correct handle implicit sext in decomposition

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.