[AArch64][SVE] Add intrinsics for first-faulting gather loads

Summary:
The following intrinsics are added:
  * @llvm.aarch64.sve.ldff1.gather
  * @llvm.aarch64.sve.ldff1.gather.index
  * @llvm.aarch64.sve.ldff1.gather_sxtw
  * @llvm.aarch64.sve.ldff1.gather.uxtw
  * @llvm.aarch64.sve.ldff1.gather_sxtw.index
  * @llvm.aarch64.sve.ldff1.gather.uxtw.index
  * @llvm.aarch64.sve.ldff1.gather.scalar.offset

Although this patch is quite substantial, the vast majority of the
implementation is just a 'copy & paste' of the implementation of regular
gather loads, including tests. There's only a handful of new
definitions:
  * AArch64ISD nodes defined in AArch64ISelLowering.h (e.g. GLDFF1)
  * Seleciton DAG Types in AArch64SVEInstrInfo.td (e.g.
    AArch64ldff1_gather)
  * intrinsics in IntrinsicsAArch64.td (e.g. aarch64_sve_ldff1_gather)
  * Pseudo instructions in SVEInstrFormats.td to workaround the issue of
    use-before-def for the FFR register.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D75128