Move fixed length SDIV tests from sve-fixed-length-int-arith.ll to sve-fixed-length-int-div.ll. The former uses CHECK lines that verify legalization decisions. That's overkill for the i8/i16 SDIV tests, since they have a tricky legalization.
There are no nxv16i8/nxv8i16 SDIV instructions, so these fixed width operations must be promoted to nxv4i32.
Differential Revision: https://reviews.llvm.org/D86114