[LiveDebugValues][NFC] Silence an unused variable warning

On release builds, 'MI' isn't used by anything (it's already inserted into a
block by BuildMI), while on non-release builds it's used by a LLVM_DEBUG
statement. Mark as explicitly used to avoid the warning.

llvm-svn: 370870

lib/CodeGen/LiveDebugValues.cpp

@@ -1172,6 +1172,7 @@ void LiveDebugValues::flushPendingLocs(VarLocInMBB &PendingInLocs,
                      DebugInstr->getDesc(), IsIndirect, Reg,
                      DebugInstr->getDebugVariable(), DebugExpr);
       }
+      (void)MI;
       LLVM_DEBUG(dbgs() << "Inserted: "; MI->dump(););
     }
   }