PPC: Add a isConsecutiveLS utility function

isConsecutiveLS is a slightly more general form of
SelectionDAG::isConsecutiveLoad. Aside from also handling stores, it also does
not assume equality of the chain operands is necessary. In the case of the PPC
backend, this chain condition is checked in a more general way by the
surrounding code.

Mostly, this part of the refactoring in preparation for supporting optimized
unaligned stores.