[X86][SSE] Allow PACKSS to be used to truncate any type of all/none sign bits input

At the moment we only use truncateVectorCompareWithPACKSS with direct vector comparison results (just one example of a known all/none signbits input).

This change relaxes the direct matching of a SETCC opcode by moving the logic up into SelectionDAG::ComputeNumSignBits and accepting any input with a known splatted signbit.

llvm-svn: 287535