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getCompare returns true, false or undef constants if the comparison can be evaluated, or nullptr if it cannot. This is in line with what ConstantExpr::getCompare returns. It also allows us to use ConstantExpr::getCompare for comparing constants. Reviewers: davide, mssimpso, dberlin, anna Reviewed By: davide Differential Revision: https://reviews.llvm.org/D43761 llvm-svn: 326720