[AArch64][SVE] Update names and comments for gathers/scatters (NFC)

Summary:
This patch renames functions and TableGen classes for SVE gathers and
scatters. The original names implied that the corresponding
methods/classes are only suited for regular gathers/scatters (i.e. LD1
and ST1), which is not the case. Indeed, we will be re-using them for
non-temporal and first-faulting gathers/scatters in the forthcoming
patches. The new names also highlight the split into Vector-Scalar (VS)
and Scalar-Vector (SV) cases.

List of changes:
* `performLD1GatherCombine` and `performST1ScatterCombine` are renamed
  as `performGatherLoadCombine` and `performScatterStoreCombine`,
  respectively.
* Selection DAG types for scatters and gathers from
  AArch64SVEInstrInfo.td are renamed. For example, `SDT_AArch64_GLD1` is
  renamed as `SDT_AArch64_GATHER_SV`. SV stands for Scalar-Vector, as
  opposed to Vector-Scalar (VS).
* The intrinsic classes from IntrinsicsAArch64.td are renamed. For
  example, `AdvSIMD_GatherLoad_64bitOffset_Intrinsic` is renamed as
  `AdvSIMD_GatherLoad_SV_64b_Offsets_Intrinsic`.
* Updated comments in `performGatherLoadCombine` and
  `performScatterStoreCombine`.

Reviewers: sdesmalen, rengolin, efriedma

Reviewed By: sdesmalen

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75035