[X86][SSE] combineBitcastvxi1 - use PACKSSWB directly to pack v8i16 to v16i8

Avoid difficulties determining the number of sign bits later on in shuffle lowering to lower to PACKSS

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316383 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed