[ARM][X86][NFC] Use lambda to avoid duplicate switches in areLoadsFromSameBasePtr (#72376)

Both the Arm and X86 implementations of areLoadsFromSameBasePtr use a
switch over the machine opcode, and repeat the same logic for both
SDNode operands. We can avoid the duplicated logic (especially lengthy
in the X86 case) by just using a lambda. This could obviously be a
candidate for moving out to a separate helper function if there were
other users, but I've made the minimal change in this patch.