[X86][XOP] Tidyup VPHADD/VPHSUB unary horizontal ops default schedule class

Based off Agner and AMD SoG tables, the XOP VPHADD/VPHSUB unary horizontal ops are as fast as basic arithmetic ops, not the slower SSSE3 binary horizontal add/sub ops. This also matches what the bdver2 model already lists.

Noticed while investigating reduction add optimizations.