There are no nxv16i8/nxv8i16 SDIV instructions, so these fixed width operations must be promoted to nxv4i32. This required adding an OverrideNEON flag to LowerToScalableOp(...), so that we can make use of the existing scalable lowering for the smaller vectors (e.g. v8i8).
Also notice that a new test file was needed. The existing SDIV tests live in sve-fixed-length-int-arith.ll, which uses the legalization-style FileCheck macros. But the complicated lowering of these operations really needs the "end result"-style macros. If this patch is accepted, I'll move the vXi32/vXi64 tests to this new file as well.
I just wanted to double check that you are aware this is going to result in i8/i16 fixed length vector divides being different to the i32/i64 ones. The latter being predicated with the former cases not (or rather using an "all true" predicate).
Given divides are rarely cheap I prefer the predicated route but I guess there's no reason to be consistent at this stage.