[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
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