[TTI] Unify FavorPostInc and FavorBackedgeIndex into getPreferredAddressingMode

This refactors shouldFavorPostInc() and shouldFavorBackedgeIndex() into
getPreferredAddressingMode() so that we have one interface to steer LSR in
generating the preferred addressing mode.

Differential Revision: https://reviews.llvm.org/D96600

GitOrigin-RevId: cd6de0e8de4a5fd558580be4b1a07116914fc8ed
8 files changed