[WebAssembly] Add an assert to sanity-check dead flags.

The WebAssemblyStoreResults pass runs before LiveVariables, so it doesn't
expect to have to keep dead flags up to date; check this with an assert.

llvm-svn: 255551

lib/Target/WebAssembly/WebAssemblyStoreResults.cpp

@@ -113,6 +113,9 @@ bool WebAssemblyStoreResults::runOnMachineFunction(MachineFunction &MF) {
           DEBUG(dbgs() << "Setting operand " << O << " in " << *Where
                        << " from " << MI << "\n");
           O.setReg(ToReg);
+          // If the store's def was previously dead, it is no longer. But the
+          // dead flag shouldn't be set yet.
+          assert(!MI.getOperand(0).isDead() && "Dead flag set on store result");
         }
       }
   }