[flang][CodeGen][NFC] Reduce PreCGRewrite pass boilerplate (#94329)

The pass constructor can be generated automatically by tablegen.

This pass is module-level and runs on all instances of target operations
inside of it and so does not need any modification to support
alternative top-level operations.

Author: tblah

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

@@ -43,7 +43,6 @@ def CodeGenRewrite : Pass<"cg-rewrite", "mlir::ModuleOp"> {
   let description = [{
     Fuse specific subgraphs into single Ops for code generation.
   }];
-  let constructor = "::fir::createFirCodeGenRewritePass()";
   let dependentDialects = [
     "fir::FIROpsDialect", "fir::FIRCodeGenDialect"
   ];

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

@@ -28,11 +28,6 @@ struct NameUniquer;
 #define GEN_PASS_DECL_BOXEDPROCEDUREPASS
 #include "flang/Optimizer/CodeGen/CGPasses.h.inc"
 
-/// Prerequiste pass for code gen. Perform intermediate rewrites to perform
-/// the code gen (to LLVM-IR dialect) conversion.
-std::unique_ptr<mlir::Pass> createFirCodeGenRewritePass(
-    CodeGenRewriteOptions Options = CodeGenRewriteOptions{});
-
 /// FirTargetRewritePass options.
 struct TargetRewriteOptions {
   bool noCharacterConversion{};

flang/include/flang/Tools/CLOptions.inc

@@ -178,7 +178,7 @@ inline void addCodeGenRewritePass(mlir::PassManager &pm, bool preserveDeclare) {
   fir::CodeGenRewriteOptions options;
   options.preserveDeclare = preserveDeclare;
   addPassConditionally(pm, disableCodeGenRewrite,
-      [&]() { return fir::createFirCodeGenRewritePass(options); });
+      [&]() { return fir::createCodeGenRewrite(options); });
 }
 
 inline void addTargetRewritePass(mlir::PassManager &pm) {

flang/lib/Optimizer/CodeGen/PreCGRewrite.cpp

@@ -327,7 +327,8 @@ class DummyScopeOpConversion
 
 class CodeGenRewrite : public fir::impl::CodeGenRewriteBase<CodeGenRewrite> {
 public:
-  CodeGenRewrite(fir::CodeGenRewriteOptions opts) : Base(opts) {}
+  using CodeGenRewriteBase<CodeGenRewrite>::CodeGenRewriteBase;
+
   void runOnOperation() override final {
     mlir::ModuleOp mod = getOperation();
 
@@ -361,11 +362,6 @@ class CodeGenRewrite : public fir::impl::CodeGenRewriteBase<CodeGenRewrite> {
 
 } // namespace
 
-std::unique_ptr<mlir::Pass>
-fir::createFirCodeGenRewritePass(fir::CodeGenRewriteOptions Options) {
-  return std::make_unique<CodeGenRewrite>(Options);
-}
-
 void fir::populatePreCGRewritePatterns(mlir::RewritePatternSet &patterns,
                                        bool preserveDeclare) {
   patterns.insert<EmboxConversion, ArrayCoorConversion, ReboxConversion,