[ARM] Fix incorrect assignment of Changed variable in MVEGatherScatterLowering::optimiseOffsets.

I believe this Changed flag should be initialized to false,
otherwise the if (!Changed) is always dead. This doesn't
manifest in a functional issue because the PHINode checks will
fail if nothing changed. They are identical to the earlier
checks that must have already failed to get into this else block.

While there remove an else after return to reduce indentation.

Differential Revision: https://reviews.llvm.org/D105159