[FIR] Avoid generating llvm.undef for dummy scoping info (#128098)

Dummy scoping operations are generated to keep track of scopes for
purpose of Fortran level analyses like Alias Analysis. For codegen, the
scoping info is converted to a fir.undef during pre-codegen rewrite.
Then during declare lowering, this info is no longer used - but it is
still translated to llvm.undef. I cleaned up so it is simply erased. The
generated LLVM should now no longer have a stray undef which looks off
when trying to make sense of the IR.

Co-authored-by: Razvan Lupusoru <rlupusoru@nvidia.com>

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -3575,6 +3575,14 @@ struct UndefOpConversion : public fir::FIROpConversion<fir::UndefOp> {
   llvm::LogicalResult
   matchAndRewrite(fir::UndefOp undef, OpAdaptor,
                   mlir::ConversionPatternRewriter &rewriter) const override {
+    if (mlir::isa<fir::DummyScopeType>(undef.getType())) {
+      // Dummy scoping is used for Fortran analyses like AA. Once it gets to
+      // pre-codegen rewrite it is erased and a fir.undef is created to
+      // feed to the fir declare operation. Thus, during codegen, we can
+      // simply erase is as it is no longer used.
+      rewriter.eraseOp(undef);
+      return mlir::success();
+    }
     rewriter.replaceOpWithNewOp<mlir::LLVM::UndefOp>(
         undef, convertType(undef.getType()));
     return mlir::success();