[BOLT] Fix warnings

This patch fixes:

  bolt/lib/Passes/PAuthGadgetScanner.cpp:564:19: error: unused
  variable 'CheckedReg' [-Werror,-Wunused-variable]

  bolt/lib/Passes/PAuthGadgetScanner.cpp:565:17: error: unused
  variable 'FirstInst' [-Werror,-Wunused-variable]

Author: kazutakahirata

bolt/lib/Passes/PAuthGadgetScanner.cpp

@@ -571,6 +571,8 @@ class DataflowSrcSafetyAnalysis
           traceInst(BC, "First instruction", FirstInst);
           traceInst(BC, "Last instruction", LastInst);
         });
+        (void)CheckedReg;
+        (void)FirstInst;
         assert(llvm::any_of(BB, [&](MCInst &I) { return &I == &FirstInst; }) &&
                "Data-flow analysis expects the checker not to cross BBs");
         CheckerSequenceInfo[&LastInst] = *CheckerInfo;