CmpPredicate::getMatching implicitly assumes that both predicates are
integer-predicates, and this has led to a crash being reported in
VectorCombine after e409204 (VectorCombine: teach foldExtractedCmps
about samesign). FP predicates are simple enough to handle as there is
never any samesign information associated with them: hence handle them
in CmpPredicate::getMatching, fixing the VectorCombine crash and
guarding against future incorrect usages.
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