[RewriteStatepointsForGC] Move an expensive debugging check to XDEBUG

The check in question is attempting to help find cases where we haven't relocated a pointer at a safepoint we should have.  It does this by coercing the value to null at any safepoint which doesn't relocate it.  

Unfortunately, this turns out to be rather expensive in terms of memory usage and time.  The number of stores inserted can grow with O(number of values x number of statepoints).  On at least one example I looked at, over half of peak memory usage was coming from this check.  

With this change, the check is no longer enabled by default in Asserts builds.  It is enabled for expensive asserts builds and has a command line option to enable it in both Asserts and non-Asserts builds.  

llvm-svn: 234761