[Statepoints][NFC] Add Statepoint::getGCResult.

[sanjoy]

Summary

Introduce a simple accessor to get the gc_result hanging off of a
statepoint.

(I don't think this change needs review, this was uploaded to
phabricator to provide context for later dependent changes.)

[reames]

LGTM

include/llvm/IR/Statepoint.h

public:
   }
 
   /// Get list of all gc reloactes linked to this statepoint
   /// May contain several relocations for the same base/derived pair.
   /// For example this could happen due to relocations on unwinding
   /// path of invoke.
   std::vector<GCRelocateOperands> getRelocates();
 
+  /// Get the experimental_gc_result call tied to this statepoint.  Can be
+  /// nullptr if there isn't a gc_result tied to this statepoint.  Guaranteed to
+  /// be a CallInst if non-null.
+  InstructionTy *getGCResult() {
+    for (auto *U : getCallSite().getInstruction()->users())
+      if (isGCResult(U))
+        return cast<CallInst>(U);
+
+    return nullptr;
+  }
+
 #ifndef NDEBUG
   /// Asserts if this statepoint is malformed.  Common cases for failure
   /// include incorrect length prefixes for variable length sections or
   /// illegal values for parameters.
   void verify() {
     assert(getNumCallArgs() >= 0 &&
            "number of arguments to actually callee can't be negative");