[StandardInstrumentations] Only turn on -verify-cfg-preserved by default in expensive checks builds

https://llvm-compile-time-tracker.com/compare.php?from=3fd42f50d8aadb4d0c348ac17cd2115c1b0564a4&to=50c37f6fc62a1e7bb4f0e307c89f760d42dbe4e9&stat=instructions:u
shows that this is fairly expensive, 5-10% increase in compile time, and I'd like to add more similar checks under the same flag.

This matches the legacy pass manager.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D146068

Author: aeubanks

llvm/lib/Passes/StandardInstrumentations.cpp

@@ -44,10 +44,10 @@
 using namespace llvm;
 
 static cl::opt<bool> VerifyPreservedCFG("verify-cfg-preserved", cl::Hidden,
-#ifdef NDEBUG
-                                        cl::init(false)
-#else
+#ifdef EXPENSIVE_CHECKS
                                         cl::init(true)
+#else
+                                        cl::init(false)
 #endif
 );