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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287535 91177308-0d34-0410-b5e6-96231b3b80d8