[Verifier] Add tied-ness verification to statepoint intsruction

[dantrushin]

LGTM

[reames]

LGTM w/changes.

Please add a verifier test which shows both failures please.

[skatkov]

Update before landing:
Upper bound check is added
verifier test is added

llvm/lib/CodeGen/MachineVerifier.cpp

case TargetOpcode::STATEPOINT: {
     };
     VerifyStackMapConstant(SO.getCCIdx());
     VerifyStackMapConstant(SO.getFlagsIdx());
     VerifyStackMapConstant(SO.getNumDeoptArgsIdx());
     VerifyStackMapConstant(SO.getNumGCPtrIdx());
     VerifyStackMapConstant(SO.getNumAllocaIdx());
     VerifyStackMapConstant(SO.getNumGcMapEntriesIdx());
 
+    // Verify that all explicit statepoint defs are tied to gc operands as
+    // they are expected to be a relocation of gc operands.
+    unsigned FirstGCPtrIdx = SO.getFirstGCPtrIdx();
+    for (unsigned Idx = 0; Idx < MI->getNumDefs(); Idx++) {
+      unsigned UseOpIdx;
+      if (!MI->isRegTiedToUseOperand(Idx, &UseOpIdx)) {
+        report("STATEPOINT defs expected to be tied", MI);
+        break;
+      }
+      if (UseOpIdx < FirstGCPtrIdx) {
+        report("STATEPOINT def tied to non-gc operand", MI);
+        break;
+      }
+    }
+
     // TODO: verify we have properly encoded deopt arguments
   } break;
   }
 }
 
 void
 MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
   const MachineInstr *MI = MO->getParent();