[BOLT] Fix no-assertions build

Summary:
In release build without assertions MCInst::dump() is undefined and
causes link time failure.

Fixes facebookarchive/BOLT#27.

(cherry picked from FBD8732905)

bolt/src/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -691,7 +691,7 @@ class AArch64MCPlusBuilder : public MCPlusBuilder {
              Inst.getOperand(0).getImm() != AArch64CC::NV &&
              "Can't reverse ALWAYS cond code");
     } else {
-      Inst.dump();
+      DEBUG(Inst.dump());
       llvm_unreachable("Unrecognized branch instruction");
     }
     return replaceBranchTarget(Inst, TBB, Ctx);