[LSR] Add a flag that overrides the target's preferred addressing mode

This adds a new flag -lsr-preferred-addressing-mode to override the target's
preferred addressing mode. It replaces flag -lsr-backedge-indexing, which is
equivalent to preindexed addressing that is one of the options that
-lsr-preferred-addressing-mode accepts.

Differential Revision: https://reviews.llvm.org/D96855