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

[aeubanks]

Summary

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.

[nikic]

LG

llvm/lib/Passes/StandardInstrumentations.cpp

 #include <unordered_map>
 #include <unordered_set>
 #include <utility>
 #include <vector>
 
 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
 );
 
 // An option that supports the -print-changed option.  See
 // the description for -print-changed for an explanation of the use
 // of this option.  Note that this option has no effect without -print-changed.
 static cl::opt<bool>
     PrintChangedBefore("print-before-changed",