[X86][SSE] combineSubToSubus - remove SSE2 early-out.

SSE2 truncation codegen has improved over the past few years (mainly due to better shuffle lowering/combining and computeKnownBits) - its no longer necessary to early-out from v8i32/v8i64 truncations.

This was noticed while looking at completing PR40111 and moving combineSubToSubus to DAGCombine entirely.