[BasicAA] Handle known bits as ranges

BasicAA currently tries to determine that the offset is positive by
checking whether all variable indices are positive based on known
bits, multiplied by a positive scale. However, this is incorrect
if the scale multiplication might overflow. In the modified test
case the original value is positive, but may be negative after a
left shift.

Fix this by converting known bits into a constant range and reusing
the range-based logic, which handles overflow correctly.

Differential Revision: https://reviews.llvm.org/D112611