[MC][SVE] Fix data operand for instruction alias of `st1d`.

The version of `st1d` that operates with vector plus immediate
addressing mode uses the alias `st1d { <Zn>.d }, <Pg>, [<Za>.d]` for
rendering `st1d { <Zn>.d }, <Pg>, [<Za>.d, #0]`. The disassembler was
generating `<Zn>.s` instead of `<Zn>.d>`.

Differential Revision: https://reviews.llvm.org/D86633