[AArch64][SVE] Handle more cases in findMoreOptimalIndexType.

This patch addresses @paulwalker-arm's comment on D117900 to
only update/write the by-ref operands iff the function returns
true. It also handles a few more cases where a series of added
offsets can be folded into the base pointer, rather than just looking
at a single offset.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D119728