[SCEV] Enable verification under EXPENSIVE_CHECKS

SCEV verification should no longer affect results of subsequent
queries, and our lit tests as well as llvm-test-suite pass with
SCEV verification enabled, so I think we can enable it by default
under EXPENSIVE_CHECKS now.

Differential Revision: https://reviews.llvm.org/D120708

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -142,8 +142,11 @@ STATISTIC(NumTripCountsNotComputed,
 STATISTIC(NumBruteForceTripCountsComputed,
           "Number of loops with trip counts computed by force");
 
-// FIXME: Enable this with EXPENSIVE_CHECKS when the test suite is clean.
+#ifdef EXPENSIVE_CHECKS
+bool llvm::VerifySCEV = true;
+#else
 bool llvm::VerifySCEV = false;
+#endif
 
 static cl::opt<unsigned>
 MaxBruteForceIterations("scalar-evolution-max-iterations", cl::ReallyHidden,