[PowerPC] Add new infrastructure to select load/store instructions, update P8/P9...

[PowerPC] Add new infrastructure to select load/store instructions, update P8/P9 load/store patterns.

This patch introduces a new infrastructure that is used to select the load and
store instructions in the PPC backend.

The primary motivation is that the current implementation of selecting load/stores
is dependent on the ordering of patterns in TableGen. Given this limitation, we
are not able to easily and reliably generate the P10 prefixed load and stores
instructions (such as when the immediates that fit within 34-bits). This
refactoring is meant to provide us with more control over the patterns/different
forms to exploit, as well as eliminating dependency of pattern declaration in TableGen.

The idea of this refactoring is that it introduces a set of addressing modes that
correspond to different instruction formats of a particular load and store
instruction, along with a set of common flags that describes a load/store.
Whenever a load/store instruction is being selected, we analyze the instruction
and compute a set of flags for it. The computed flags are then used to
select the most optimal load/store addressing mode.

This patch is the first of a series of patches to be committed - it contains the
initial implementation of the refactored load/store selection infrastructure and
also updates P8/P9 patterns to adopt this infrastructure. The idea is that
incremental patches will add more implementation and support, and eventually
the old implementation will be removed.

Differential Revision: https://reviews.llvm.org/D93370