[BasicAA] Account for wrapping when using abs(VarIndex) >= abs(Scale).

The patch adds an extra check to only set MinAbsVarIndex if
abs(V * Scale) won't wrap. In the absence of IsNSW, try to use the
bitwidths of the original V and Scale to rule out wrapping.

Attempt to model https://alive2.llvm.org/ce/z/HE8ZKj

The code in the else if below probably needs the same treatment, but I
need to come up with a test first.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D121695