[AArch64][SVE] Gather loads: pass 32 bit unpacked offsets as nxv2i32

Summary:
Currently 32 bit unpacked offsets are passed as nxv2i64. However, as
pointed out in https://reviews.llvm.org/D71074, using nxv2i32 instead
would improve consistency with:
  * how other arguments are treated
  * how scatter stores are implemented
This patch makes sure that 32 bit unpacked offsets are passes as nxv2i32
instead of nxv2i64.

Reviewers: sdesmalen, efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71724