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First half of PR40800, this patch adds DAG undef handling to icmp instructions to match the behaviour in llvm::ConstantFoldCompareInstruction and SimplifyICmpInst, this permits constant folding of vector comparisons where some elements had been reduced to UNDEF (by SimplifyDemandedVectorElts etc.). This involved a lot of tweaking to reduced tests as bugpoint loves to reduce icmp arguments to undef........ Differential Revision: https://reviews.llvm.org/D59363 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356938 91177308-0d34-0410-b5e6-96231b3b80d8