[SVE][CodeGen] Remove performMaskedGatherScatterCombine

The AArch64 DAG combine added by D90945 & D91433 extends the index
of a scalable masked gather or scatter to i32 if necessary.

This patch removes the combine and instead adds shouldExtendGSIndex, which
is used by visitMaskedGather/Scatter in SelectionDAGBuilder to query whether
the index should be extended before calling getMaskedGather/Scatter.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D94525