[flang] Avoid generating duplicate symbol in comdat (#114472)

In case where a fir.global might be duplicated in an inner module
(gpu.module), the conversion pattern will be applied on the module and
the gpu module version of the global and try to generate multiple comdat
with the same symbol name. This is what we have in the implementation of
CUDA Fortran.

Just check for the presence of the `ComdatSelectorOp` before creating a
new one.

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -2931,6 +2931,9 @@ private:
       comdatOp =
           rewriter.create<mlir::LLVM::ComdatOp>(module.getLoc(), comdatName);
     }
+    if (auto select = comdatOp.lookupSymbol<mlir::LLVM::ComdatSelectorOp>(
+            global.getSymName()))
+      return;
     mlir::OpBuilder::InsertionGuard guard(rewriter);
     rewriter.setInsertionPointToEnd(&comdatOp.getBody().back());
     auto selectorOp = rewriter.create<mlir::LLVM::ComdatSelectorOp>(

flang/test/Fir/comdat-present.fir

@@ -0,0 +1,14 @@
+// RUN: fir-opt %s --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" | FileCheck %s
+// RUN: fir-opt %s --fir-to-llvm-ir="target=x86_64-pc-windows-msvc" | FileCheck %s
+
+fir.global linkonce_odr @global_linkonce_odr constant : i32 {
+  %0 = arith.constant 0 : i32
+  fir.has_value %0 : i32
+}
+
+llvm.comdat @__llvm_comdat {
+  llvm.comdat_selector @global_linkonce_odr any
+}
+
+// CHECK-LABEL: llvm.comdat @__llvm_comdat
+// CHECK: llvm.comdat_selector @global_linkonce_odr any