[BasicAA] Don't use MinAbsVarIndex = 1. (#72993)

The current code incorrectly assumed that the absolute variable index
needs to be at least 1, if the variable is != 0. This is incorrect, in
case multiplying with Scale wraps.

The code below already checks for wrapping properly, so just remove the
incorrect assignment.

Fixes https://github.com/llvm/llvm-project/issues/72831.