Fix potential crash in Flang generateLLVMIR() when MLIR fails to translate to LLVM

[mehdi_amini]

Summary

This is pure code motion, to ensure that the check if we have a valid llvmModule
comes before trying to set option on this module.

[awarzynski]

LGTM, thanks for the fix Mehdi!

flang/lib/Frontend/FrontendActions.cpp

if (!mlir::succeeded(pm.run(*mlirModule))) {
     ci.getDiagnostics().Report(diagID);
   }
 
   // Translate to LLVM IR
   std::optional<llvm::StringRef> moduleName = mlirModule->getName();
   llvmModule = mlir::translateModuleToLLVMIR(
       *mlirModule, *llvmCtx, moduleName ? *moduleName : "FIRModule");
 
+  if (!llvmModule) {
+    unsigned diagID = ci.getDiagnostics().getCustomDiagID(
+        clang::DiagnosticsEngine::Error, "failed to create the LLVM module");
+    ci.getDiagnostics().Report(diagID);
+    return;
+  }
+
   // Set PIC/PIE level LLVM module flags.
   if (opts.PICLevel > 0) {
     llvmModule->setPICLevel(static_cast<llvm::PICLevel::Level>(opts.PICLevel));
     if (opts.IsPIE)
       llvmModule->setPIELevel(
           static_cast<llvm::PIELevel::Level>(opts.PICLevel));
   }
 
-  if (!llvmModule) {
-    unsigned diagID = ci.getDiagnostics().getCustomDiagID(
-        clang::DiagnosticsEngine::Error, "failed to create the LLVM module");
-    ci.getDiagnostics().Report(diagID);
-    return;
-  }
 }
 
 void CodeGenAction::setUpTargetMachine() {
   CompilerInstance &ci = this->getInstance();
 
   const TargetOptions &targetOpts = ci.getInvocation().getTargetOpts();
   const std::string &theTriple = targetOpts.triple;