[AArch64][SVE] Rename intrinsics for gather prefetch [NFC]

Summary:
In order to keep the names consistent with other SVE gather loads, the
intrinsics for gather prefetch are renamed as follows:
  * @llvm.aarch64.sve.gather.prfb -> @llvm.aarch64.sve.prfb.gather

Reviewed by: fpetrogalli

Differential Revision: https://reviews.llvm.org/D76421