Add `mlirModuleFromOperation` to C API

[apaszke]

Summary

At the moment MlirModules can be converted to MlirOperations, but not
the other way around (at least not without going around the C API). This
makes it impossible to e.g. run passes over a ModuleOp created through
mlirOperationCreate.

[mehdi_amini]

Can you add a C unit-test with this?

[apaszke]

Added tests, as requested

[mehdi_amini]

Thanks!

mlir/include/mlir-c/IR.h

 static inline bool mlirModuleIsNull(MlirModule module) { return !module.ptr; }
 
 /// Takes a module owned by the caller and deletes it.
 MLIR_CAPI_EXPORTED void mlirModuleDestroy(MlirModule module);
 
 /// Views the module as a generic operation.
 MLIR_CAPI_EXPORTED MlirOperation mlirModuleGetOperation(MlirModule module);
 
+/// Views the generic operation as a module.
+/// The returned module is null when the input operation was not a ModuleOp.
+MLIR_CAPI_EXPORTED MlirModule mlirModuleFromOperation(MlirOperation op);
+
 //===----------------------------------------------------------------------===//
 // Operation state.
 //===----------------------------------------------------------------------===//
 
 /// An auxiliary class for constructing operations.
 ///
 /// This class contains all the information necessary to construct the
 /// operation. It owns the MlirRegions it has pointers to and does not own

mlir/lib/CAPI/IR/IR.cpp

void mlirModuleDestroy(MlirModule module) {
   // Transfer ownership to an OwningModuleRef so that its destructor is called.
   OwningModuleRef(unwrap(module));
 }
 
 MlirOperation mlirModuleGetOperation(MlirModule module) {
   return wrap(unwrap(module).getOperation());
 }
 
+MlirModule mlirModuleFromOperation(MlirOperation op) {
+  return wrap(dyn_cast<ModuleOp>(unwrap(op)));
+}
+
 //===----------------------------------------------------------------------===//
 // Operation state API.
 //===----------------------------------------------------------------------===//
 
 MlirOperationState mlirOperationStateGet(MlirStringRef name, MlirLocation loc) {
   MlirOperationState state;
   state.name = name;
   state.location = loc;

mlir/test/CAPI/ir.c

static void printFirstOfEach(MlirContext ctx, MlirOperation operation) {
   // function.
   MlirRegion region = mlirOperationGetRegion(operation, 0);
   MlirBlock block = mlirRegionGetFirstBlock(region);
   operation = mlirBlockGetFirstOperation(block);
   region = mlirOperationGetRegion(operation, 0);
   MlirOperation parentOperation = operation;
   block = mlirRegionGetFirstBlock(region);
   operation = mlirBlockGetFirstOperation(block);
+  assert(mlirModuleIsNull(mlirModuleFromOperation(operation)));
 
   // Verify that parent operation and block report correctly.
   fprintf(stderr, "Parent operation eq: %d\n",
           mlirOperationEqual(mlirOperationGetParentOperation(operation),
                              parentOperation));
   fprintf(stderr, "Block eq: %d\n",
           mlirBlockEqual(mlirOperationGetBlock(operation), block));
   // CHECK: Parent operation eq: 1
static void printFirstOfEach(MlirContext ctx, MlirOperation operation) {
   mlirOpPrintingFlagsDestroy(flags);
 }
 
 static int constructAndTraverseIr(MlirContext ctx) {
   MlirLocation location = mlirLocationUnknownGet(ctx);
 
   MlirModule moduleOp = makeAndDumpAdd(ctx, location);
   MlirOperation module = mlirModuleGetOperation(moduleOp);
+  assert(!mlirModuleIsNull(mlirModuleFromOperation(module)));
 
   int errcode = collectStats(module);
   if (errcode)
     return errcode;
 
   printFirstOfEach(ctx, module);
 
   mlirModuleDestroy(moduleOp);