[Utils] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Utils/SimplifyCFG.cpp:338:6: error: unused
  function 'isSelectInRoleOfConjunctionOrDisjunction'
  [-Werror,-Wunused-function]

Author: kazutakahirata

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -335,7 +335,8 @@ class SimplifyCFGOpt {
 // we synthesize a || b as select a, true, b
 // we synthesize a && b as select a, b, false
 // this function determines if SI is playing one of those roles.
-bool isSelectInRoleOfConjunctionOrDisjunction(const SelectInst *SI) {
+[[maybe_unused]] bool
+isSelectInRoleOfConjunctionOrDisjunction(const SelectInst *SI) {
   return ((isa<ConstantInt>(SI->getTrueValue()) &&
            (dyn_cast<ConstantInt>(SI->getTrueValue())->isOne())) ||
           (isa<ConstantInt>(SI->getFalseValue()) &&