Unify XDEBUG and EXPENSIVE_CHECKS (into the latter), and add an option to the cmake build to enable them.

Summary:
Historically, we had a switch in the Makefiles for turning on "expensive
checks". This has never been ported to the cmake build, but the
(dead-ish) code is still around.

This will also make it easier to turn it on in buildbots.

Reviewers: chandlerc

Subscribers: jyknight, mzolotukhin, RKSimon, gberry, llvm-commits

Differential Revision: http://reviews.llvm.org/D19723

llvm-svn: 268050

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -111,7 +111,7 @@ MaxBruteForceIterations("scalar-evolution-max-iterations", cl::ReallyHidden,
                                  "derived loop"),
                         cl::init(100));
 
-// FIXME: Enable this with XDEBUG when the test suite is clean.
+// FIXME: Enable this with EXPENSIVE_CHECKS when the test suite is clean.
 static cl::opt<bool>
 VerifySCEV("verify-scev",
            cl::desc("Verify ScalarEvolution's backedge taken counts (slow)"));