[Statepoint] Replace uses of isX functions with idiomatic isa<X>

Now that all of the statepoint related routines have classes with isa support, let's cleanup.

I'm leaving the (dead) utitilities in tree for a few days so that I can do the same cleanup downstream without breakage.
diff --git a/llvm/lib/IR/SafepointIRVerifier.cpp b/llvm/lib/IR/SafepointIRVerifier.cpp
index c9aa85b..01c0132 100644
--- a/llvm/lib/IR/SafepointIRVerifier.cpp
+++ b/llvm/lib/IR/SafepointIRVerifier.cpp
@@ -783,7 +783,7 @@
 
 void GCPtrTracker::transferInstruction(const Instruction &I, bool &Cleared,
                                        AvailableValueSet &Available) {
-  if (isStatepoint(I)) {
+  if (isa<GCStatepointInst>(I)) {
     Cleared = true;
     Available.clear();
   } else if (containsGCPtrType(I.getType()))