[X86][SSE] combineVectorSignBitsTruncation - fold trunc(srl(x,c)) -> packss(sra(x,c))

If a srl doesn't introduce any sign bits into the truncated result, then replace with a sra to let us use a PACKSS truncation - fixes a regression noticed in D56387 on pre-SSE41 targets that don't have PACKUSDW.