[AArch64][SVE] Test more types in sve-fixed-length-subvector.ll

Previously only the i32 type was tested. Now, the {i,f}{16,32,64} types
are tested.

The v8{i,f}16 cases lower differently to the other cases, which is worth
defending. The lowering for the other cases is currently identical, but
probably worth having for the better coverage.

Differential Revision: https://reviews.llvm.org/D98690