[SCEV] Enable verification under EXPENSIVE_CHECKS

[nikic]

Summary

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.

Depends on D120551.

[reames]

LGTM

I will be very curious what falls out once you land this. :)

llvm/lib/Analysis/ScalarEvolution.cpp

STATISTIC(NumTripCountsComputed,
           "Number of loops with predictable loop counts");
 STATISTIC(NumTripCountsNotComputed,
           "Number of loops without predictable loop counts");
 STATISTIC(NumBruteForceTripCountsComputed,
           "Number of loops with trip counts computed by force");
 STATISTIC(NumFoundPhiSCCs,
           "Number of found Phi-composed strongly connected components");
 
-// 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,
                         cl::ZeroOrMore,
                         cl::desc("Maximum number of iterations SCEV will "
                                  "symbolically execute a constant "
                                  "derived loop"),
                         cl::init(100));