[X86] lowerV8I16Shuffle - use PACKSS(SEXT_INREG(X),SEXT_INREG(Y)) for pre-SSSE3 truncation shuffles

The comment about PSHUFLW+PSHUFHW+PSHUFD was outdated as that referred to a single input case, but that is now always handled earlier.

Another step towards removing premature combines to vector truncation combines to PACK.