[flang][codegen] Add `fir.end` conversion placeholder

This patch extends the FIRToLLVMLowering pass in Flang by adding a hook
to transform fir.end. This is just a placeholder for now as fir.end is
not required yet.

This is part of the upstreaming effort from the `fir-dev` branch in [1].

[1] https://github.com/flang-compiler/f18-llvm-project

Patch originally written by:
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

Differential Revision: https://reviews.llvm.org/D113295

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -839,6 +839,18 @@ struct GenTypeDescOpConversion : public FIROpConversion<fir::GenTypeDescOp> {
   }
 };
 
+/// Convert `fir.end`
+struct FirEndOpConversion : public FIROpConversion<fir::FirEndOp> {
+  using FIROpConversion::FIROpConversion;
+
+  mlir::LogicalResult
+  matchAndRewrite(fir::FirEndOp firEnd, OpAdaptor,
+                  mlir::ConversionPatternRewriter &rewriter) const override {
+    return rewriter.notifyMatchFailure(
+        firEnd, "fir.end codegen is not implemented yet");
+  }
+};
+
 /// Lower `fir.has_value` operation to `llvm.return` operation.
 struct HasValueOpConversion : public FIROpConversion<fir::HasValueOp> {
   using FIROpConversion::FIROpConversion;
@@ -2108,9 +2120,9 @@ public:
         CmpcOpConversion, ConstcOpConversion, ConvertOpConversion,
         DispatchOpConversion, DispatchTableOpConversion, DTEntryOpConversion,
         DivcOpConversion, EmboxOpConversion, EmboxCharOpConversion,
-        EmboxProcOpConversion, ExtractValueOpConversion, HasValueOpConversion,
-        GenTypeDescOpConversion, GlobalLenOpConversion, GlobalOpConversion,
-        InsertOnRangeOpConversion, InsertValueOpConversion,
+        EmboxProcOpConversion, ExtractValueOpConversion, FirEndOpConversion,
+        HasValueOpConversion, GenTypeDescOpConversion, GlobalLenOpConversion,
+        GlobalOpConversion, InsertOnRangeOpConversion, InsertValueOpConversion,
         IsPresentOpConversion, LoadOpConversion, NegcOpConversion,
         MulcOpConversion, SelectCaseOpConversion, SelectOpConversion,
         SelectRankOpConversion, SelectTypeOpConversion, ShapeOpConversion,

flang/test/Fir/convert-to-llvm-invalid.fir

@@ -173,3 +173,10 @@ func @emboxproc_test() {
 
 // Test that `fir.unboxproc` and `fir.boxproc_host` also fails to be legalized.
 // At the moment these cannot be tested since the `fir.boxproc` type does not have a conversion.
+
+// -----
+
+// Test `fir.end` conversion failure. Not implemented yet.
+
+// expected-error@+1{{failed to legalize operation 'fir.end'}}
+"fir.end"() : () -> ()