[build] Update llvm tag to 68754241 (#2079)

This commit updates the `llvm-project` and `mlir-hlo` submodules to
commits:

- llvm-project: 6875424135312aeb26ab8e0358ba7f9e6e80e741
- mlir-hlo: 92fd33a4bacbeb93ab276a49f38bdebd5f9d7487

The calls to `mlir::MlirOptMain` are updated to no longer specify the
flag `preloadDialectInContext` that has been removed (see:
https://reviews.llvm.org/D149039).

externals/llvm-external-projects/torch-mlir-dialects/tools/torch-mlir-dialects-opt/torch-mlir-dialects-opt.cpp

@@ -43,7 +43,6 @@ int main(int argc, char **argv) {
       mlir::func::FuncDialect, mlir::memref::MemRefDialect,
       mlir::scf::SCFDialect, mlir::tensor::TensorDialect>();
 
-  return mlir::asMainReturnCode(
-      mlir::MlirOptMain(argc, argv, "MLIR modular optimizer driver\n", registry,
-                        /*preloadDialectsInContext=*/false));
+  return mlir::asMainReturnCode(mlir::MlirOptMain(
+      argc, argv, "MLIR modular optimizer driver\n", registry));
 }

externals/llvm-project

@@ -1 +1 @@
-Subproject commit 3157f03a349cfc852cdd994675eaa9652caa2e3a
+Subproject commit 6875424135312aeb26ab8e0358ba7f9e6e80e741

externals/mlir-hlo

@@ -1 +1 @@
-Subproject commit a9ac343738945be7744e304b72704128e48aa06f
+Subproject commit 92fd33a4bacbeb93ab276a49f38bdebd5f9d7487

tools/torch-mlir-opt/torch-mlir-opt.cpp

@@ -28,8 +28,7 @@ int main(int argc, char **argv) {
   
 #ifdef TORCH_MLIR_ENABLE_STABLEHLO
   mlir::stablehlo::registerAllDialects(registry);
-#endif  
-  return mlir::asMainReturnCode(
-      mlir::MlirOptMain(argc, argv, "MLIR modular optimizer driver\n", registry,
-                        /*preloadDialectsInContext=*/false));
+#endif
+  return mlir::asMainReturnCode(mlir::MlirOptMain(
+      argc, argv, "MLIR modular optimizer driver\n", registry));
 }