[SPIRV] Fix -Wunused-but-set-variable. NFC

Author: MaskRay

llvm/lib/Target/SPIRV/SPIRVMergeRegionExitTargets.cpp

@@ -106,7 +106,7 @@ class SPIRVMergeRegionExitTargets : public FunctionPass {
                             const std::unordered_set<BasicBlock *> ToReplace,
                             BasicBlock *NewTarget) {
     auto *T = BB->getTerminator();
-    if (auto *RI = dyn_cast<ReturnInst>(T))
+    if (isa<ReturnInst>(T))
       return;
 
     if (auto *BI = dyn_cast<BranchInst>(T)) {