[AArch64][ISel] Always use pre-inc/post-inc addressing mode for auto-indexed...

[AArch64][ISel] Always use pre-inc/post-inc addressing mode for auto-indexed load/store with constant offset.

Unlike ARM target, current AArch64 target doesn't have facility to encode the
operation bit: whether to add an offset to base pointer for pre-inc/post-inc
addressing mode, or to subtract an offset from base pointer for
pre-dec/post-dec addressing mode.

A mis-compile (https://github.com/llvm/llvm-project/issues/60645) was noticed
due to this limitation.

Therefore, for AArch64 auto-indexed load/store with constant offset, always
use pre-inc/post-inc addressing mode. The constant offset is negated for
pre-dec/post-dec addressing mode.
An auto-indexed address with non-constant offset is currently not split into
base and offset parts. If we are to handle non-constant offset in the future,
offset node will need to take a negate.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D143796