Fix unused variable from r240792.

The variable 'I' wasn't used when assertions were disabled.
This commit ensures that 'I' is used outside of an assert.

llvm-svn: 240797
diff --git a/llvm/lib/CodeGen/MIRPrinter.cpp b/llvm/lib/CodeGen/MIRPrinter.cpp
index f9595f8..9446849 100644
--- a/llvm/lib/CodeGen/MIRPrinter.cpp
+++ b/llvm/lib/CodeGen/MIRPrinter.cpp
@@ -90,6 +90,7 @@
     // map back to the correct MBBs when parsing the output.
     assert(MBB.getNumber() == I++ &&
            "Can't print MBBs that aren't sequentially numbered");
+    (void)I;
     yaml::MachineBasicBlock YamlMBB;
     convert(YamlMBB, MBB);
     YamlMF.BasicBlocks.push_back(YamlMBB);