[flang] Add changes to codegen to convert it to tablegen passes.

[schweitz]

Summary

This upstreams changes to codegen to convert the passes to the new
tablegen pass support from MLIR.

[clementval]

LGTM

flang/include/flang/Optimizer/CMakeLists.txt

+add_subdirectory(CodeGen)
 add_subdirectory(Dialect)

flang/include/flang/Optimizer/CodeGen/CGPasses.td

+//===-- CGPasses.td - code gen pass definition file --------*- tablegen -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains definitions for passes within the Optimizer/CodeGen/
+//  directory.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FLANG_OPTIMIZER_CODEGEN_PASSES
+#define FLANG_OPTIMIZER_CODEGEN_PASSES
+
+include "mlir/Pass/PassBase.td"
+
+def CodeGenRewrite : FunctionPass<"cg-rewrite"> {
+  let summary = "Rewrite some FIR ops into their code-gen forms.";
+  let constructor = "fir::createFirCodeGenRewritePass()";
+}
+
+#endif // FLANG_OPTIMIZER_CODEGEN_PASSES

flang/include/flang/Optimizer/CodeGen/CMakeLists.txt

+
+set(LLVM_TARGET_DEFINITIONS CGPasses.td)
+mlir_tablegen(CGPasses.h.inc -gen-pass-decls)
+add_public_tablegen_target(FIROptCodeGenPassIncGen)
+
+add_mlir_doc(Passes -gen-pass-doc OptimizerCodeGenPasses ./)

flang/include/flang/Optimizer/CodeGen/CodeGen.h

 //===-- Optimizer/CodeGen/CodeGen.h -- code generation ----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef OPTIMIZER_CODEGEN_CODEGEN_H
 #define OPTIMIZER_CODEGEN_CODEGEN_H
 
+#include "mlir/Pass/Pass.h"
+#include "mlir/Pass/PassRegistry.h"
 #include <memory>
 
-namespace llvm {
-class raw_ostream;
-}
-namespace mlir {
-class Pass;
-}
-
 namespace fir {
 
 struct NameUniquer;
 
+/// Prerequiste pass for code gen. Perform intermediate rewrites to perform
+/// the code gen (to LLVM-IR dialect) conversion.
+std::unique_ptr<mlir::Pass> createFirCodeGenRewritePass();
+
 /// Convert FIR to the LLVM IR dialect
 std::unique_ptr<mlir::Pass> createFIRToLLVMPass(NameUniquer &uniquer);
 
 /// Convert the LLVM IR dialect to LLVM-IR proper
 std::unique_ptr<mlir::Pass>
 createLLVMDialectToLLVMPass(llvm::raw_ostream &output);
 
+inline void registerOptCodeGenPasses() {
+  using mlir::Pass;
+// declarative passes
+#define GEN_PASS_REGISTRATION
+#include "flang/Optimizer/CodeGen/CGPasses.h.inc"
+}
+
 } // namespace fir
 
 #endif // OPTIMIZER_CODEGEN_CODEGEN_H