[ARM] Correct predicate operand for offset gather/scatter

These arm_mve_vldr_gather_offset_predicated and
arm_mve_vstr_scatter_offset_predicated have some extra parameters
meaning the predicate is at a later operand. If a loop contains _only_
those masked instructions, we would miss transforming the active lane
mask.

Differential Revision: https://reviews.llvm.org/D86791