[flang][CodeGen] Avoid out-of-bounds memory access in SelectCaseOp (#92955)

`SelectCaseOp::getCompareOperands` may return an empty range for the
"default" case. Do not dereference the range until it is expected to be
non-empty.

This was detected by address-sanitizer.

Author: kparzysz

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -2981,39 +2981,40 @@ struct SelectCaseOpConversion : public fir::FIROpConversion<fir::SelectCaseOp> {
           caseOp.getSuccessorOperands(adaptor.getOperands(), t);
       std::optional<mlir::ValueRange> cmpOps =
           *caseOp.getCompareOperands(adaptor.getOperands(), t);
-      mlir::Value caseArg = *(cmpOps.value().begin());
       mlir::Attribute attr = cases[t];
+      assert(mlir::isa<mlir::UnitAttr>(attr) || cmpOps.has_value());
       if (mlir::isa<fir::PointIntervalAttr>(attr)) {
         auto cmp = rewriter.create<mlir::LLVM::ICmpOp>(
-            loc, mlir::LLVM::ICmpPredicate::eq, selector, caseArg);
+            loc, mlir::LLVM::ICmpPredicate::eq, selector, cmpOps->front());
         genCaseLadderStep(loc, cmp, dest, destOps, rewriter);
         continue;
       }
       if (mlir::isa<fir::LowerBoundAttr>(attr)) {
         auto cmp = rewriter.create<mlir::LLVM::ICmpOp>(
-            loc, mlir::LLVM::ICmpPredicate::sle, caseArg, selector);
+            loc, mlir::LLVM::ICmpPredicate::sle, cmpOps->front(), selector);
         genCaseLadderStep(loc, cmp, dest, destOps, rewriter);
         continue;
       }
       if (mlir::isa<fir::UpperBoundAttr>(attr)) {
         auto cmp = rewriter.create<mlir::LLVM::ICmpOp>(
-            loc, mlir::LLVM::ICmpPredicate::sle, selector, caseArg);
+            loc, mlir::LLVM::ICmpPredicate::sle, selector, cmpOps->front());
         genCaseLadderStep(loc, cmp, dest, destOps, rewriter);
         continue;
       }
       if (mlir::isa<fir::ClosedIntervalAttr>(attr)) {
-        auto cmp = rewriter.create<mlir::LLVM::ICmpOp>(
-            loc, mlir::LLVM::ICmpPredicate::sle, caseArg, selector);
+        mlir::Value caseArg0 = *cmpOps->begin();
+        auto cmp0 = rewriter.create<mlir::LLVM::ICmpOp>(
+            loc, mlir::LLVM::ICmpPredicate::sle, caseArg0, selector);
         auto *thisBlock = rewriter.getInsertionBlock();
         auto *newBlock1 = createBlock(rewriter, dest);
         auto *newBlock2 = createBlock(rewriter, dest);
         rewriter.setInsertionPointToEnd(thisBlock);
-        rewriter.create<mlir::LLVM::CondBrOp>(loc, cmp, newBlock1, newBlock2);
+        rewriter.create<mlir::LLVM::CondBrOp>(loc, cmp0, newBlock1, newBlock2);
         rewriter.setInsertionPointToEnd(newBlock1);
-        mlir::Value caseArg0 = *(cmpOps.value().begin() + 1);
-        auto cmp0 = rewriter.create<mlir::LLVM::ICmpOp>(
-            loc, mlir::LLVM::ICmpPredicate::sle, selector, caseArg0);
-        genCondBrOp(loc, cmp0, dest, destOps, rewriter, newBlock2);
+        mlir::Value caseArg1 = *(cmpOps->begin() + 1);
+        auto cmp1 = rewriter.create<mlir::LLVM::ICmpOp>(
+            loc, mlir::LLVM::ICmpPredicate::sle, selector, caseArg1);
+        genCondBrOp(loc, cmp1, dest, destOps, rewriter, newBlock2);
         rewriter.setInsertionPointToEnd(newBlock2);
         continue;
       }