[X86][SSE] Add support for lowering shuffles to PACKSS/PACKUS

If the upper bits of a truncation shuffle patterns have at least the minimum number of sign/zero bits on their inputs then we can safely use PACKSS/PACKUS as shuffles.

Partial fix for https://bugs.llvm.org/show_bug.cgi?id=34773

Differential Revision: https://reviews.llvm.org/D38472

llvm-svn: 314788