[AArch64][SVE] Workaround incorrect types when lowering fixed length gather/scatter

When lowering a fixed length gather/scatter the index type is assumed to
be the same as the memory type, this is incorrect in cases where the
extension of the index has been folded into the addressing mode.

For now add a temporary workaround to fix the codegen faults caused by
this by preventing the removal of this extension. At a later date the
lowering for SVE gather/scatters will be redesigned to improve the way
addressing modes are handled.

As a short term side effect of this change, the addressing modes
generated for fixed length gather/scatters will not be optimal.

Differential Revision: https://reviews.llvm.org/D109145