Reland "[AArch64][SVE][InstCombine] Combine contiguous gather/scatter to load/store"

This reverts commit 753eba64.

Contiguous gather => masked load:

  (sve.ld1.gather.index Mask BasePtr (sve.index IndexBase 1))
  => (masked.load (gep BasePtr IndexBase) Align Mask undef)

Contiguous scatter => masked store:

  (sve.ld1.scatter.index Value Mask BasePtr (sve.index IndexBase 1))
  => (masked.store Value (gep BasePtr IndexBase) Align Mask)

Tests with <vscale x 2 x double>:

[Gather, Scatter] for each [Positive test (index=1), Negative test
(index=2), Alignment propagation].

Differential Revision: https://reviews.llvm.org/D112076