[Clang][AArch64][SVE] Allow write to SVE vector elements using the subscript operator (#91965)

The patch at https://reviews.llvm.org/D122732 introduced using the array
subscript operator for SVE vectors, however it also causes an ICE when
the subscripting expression is used as an lvalue.

This patches fixes the error. Lvalue subscripting expressions are
emitted as LLVM IR `insertelement`.