[PowerPC] ppc64-P9-vabsd.ll - update v16i8 abdu test now that it vectorizes in the middle-end (#154712)

The scalarized IR was written before improvements to SLP / cost models
ensured that the abs intrinsic was easily vectorizable

opt -O3 : https://zig.godbolt.org/z/39T65vh8M

Now that it is we need a more useful llc test

Author: RKSimon