[flang] Migrate away from PointerUnion::{is,get} (NFC)

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.

Author: kazutakahirata

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -1613,12 +1613,13 @@ struct EmboxCommonConversion : public fir::FIROpConversion<OP> {
         if (gepArgs.size() != 1)
           fir::emitFatalError(loc,
                               "corrupted substring GEP in fir.embox/fir.rebox");
-        mlir::Type outterOffsetTy = gepArgs[0].get<mlir::Value>().getType();
+        mlir::Type outterOffsetTy =
+            llvm::cast<mlir::Value>(gepArgs[0]).getType();
         mlir::Value cast =
             this->integerCast(loc, rewriter, outterOffsetTy, *substringOffset);
 
         gepArgs[0] = rewriter.create<mlir::LLVM::AddOp>(
-            loc, outterOffsetTy, gepArgs[0].get<mlir::Value>(), cast);
+            loc, outterOffsetTy, llvm::cast<mlir::Value>(gepArgs[0]), cast);
       }
     }
     mlir::Type llvmPtrTy = ::getLlvmPtrType(resultTy.getContext());

flang/lib/Optimizer/Transforms/AddAliasTags.cpp

@@ -227,7 +227,7 @@ void AddAliasTagsPass::runOnAliasInterface(fir::FirAliasTagOpInterface op,
       source.kind == fir::AliasAnalysis::SourceKind::Argument) {
     LLVM_DEBUG(llvm::dbgs().indent(2)
                << "Found reference to dummy argument at " << *op << "\n");
-    std::string name = getFuncArgName(source.origin.u.get<mlir::Value>());
+    std::string name = getFuncArgName(llvm::cast<mlir::Value>(source.origin.u));
     if (!name.empty())
       tag = state.getFuncTreeWithScope(func, scopeOp)
                 .dummyArgDataTree.getTag(name);
@@ -240,7 +240,7 @@ void AddAliasTagsPass::runOnAliasInterface(fir::FirAliasTagOpInterface op,
   } else if (enableGlobals &&
              source.kind == fir::AliasAnalysis::SourceKind::Global &&
              !source.isBoxData()) {
-    mlir::SymbolRefAttr glbl = source.origin.u.get<mlir::SymbolRefAttr>();
+    mlir::SymbolRefAttr glbl = llvm::cast<mlir::SymbolRefAttr>(source.origin.u);
     const char *name = glbl.getRootReference().data();
     LLVM_DEBUG(llvm::dbgs().indent(2) << "Found reference to global " << name
                                       << " at " << *op << "\n");
@@ -250,8 +250,7 @@ void AddAliasTagsPass::runOnAliasInterface(fir::FirAliasTagOpInterface op,
   } else if (enableDirect &&
              source.kind == fir::AliasAnalysis::SourceKind::Global &&
              source.isBoxData()) {
-    if (source.origin.u.is<mlir::SymbolRefAttr>()) {
-      mlir::SymbolRefAttr glbl = source.origin.u.get<mlir::SymbolRefAttr>();
+    if (auto glbl = llvm::dyn_cast<mlir::SymbolRefAttr>(source.origin.u)) {
       const char *name = glbl.getRootReference().data();
       LLVM_DEBUG(llvm::dbgs().indent(2) << "Found reference to direct " << name
                                         << " at " << *op << "\n");
@@ -269,7 +268,7 @@ void AddAliasTagsPass::runOnAliasInterface(fir::FirAliasTagOpInterface op,
              source.kind == fir::AliasAnalysis::SourceKind::Allocate) {
     std::optional<llvm::StringRef> name;
     mlir::Operation *sourceOp =
-        source.origin.u.get<mlir::Value>().getDefiningOp();
+        llvm::cast<mlir::Value>(source.origin.u).getDefiningOp();
     if (auto alloc = mlir::dyn_cast_or_null<fir::AllocaOp>(sourceOp))
       name = alloc.getUniqName();
     else if (auto alloc = mlir::dyn_cast_or_null<fir::AllocMemOp>(sourceOp))

flang/lib/Optimizer/Transforms/StackArrays.cpp

@@ -76,8 +76,9 @@ class InsertionPoint {
   /// Get contained pointer type or nullptr
   template <class T>
   T *tryGetPtr() const {
-    if (location.is<T *>())
-      return location.get<T *>();
+    // Use llvm::dyn_cast_if_present because location may be null here.
+    if (T *ptr = llvm::dyn_cast_if_present<T *>(location))
+      return ptr;
     return nullptr;
   }