[X86][SSE] SimplifyDemandedBitsForTargetNode - Add initial PACKSS support

In the case where we only want the sign bit (e.g. when using PACKSS truncation of comparison results for MOVMSK) then we can just demand the sign bit of the source operands.

This makes use of the fact that PACKSS saturates out of range values to the min/max int values - so the sign bit is always preserved.

Differential Revision: https://reviews.llvm.org/D60333

llvm-svn: 357859