[AsmPrinter] Migrate away from PointerUnion::dyn_cast (NFC) (llvm#135740)

Note that PointerUnion::dyn_cast has 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>

We use dyn_cast_if_present here because Bound can be null.

Author: kazutakahirata

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -1481,16 +1481,16 @@ void DwarfUnit::constructSubrangeDIE(DIE &DW_Subrange, const DISubrangeType *SR,
 
   auto AddBoundTypeEntry = [&](dwarf::Attribute Attr,
                                DISubrangeType::BoundType Bound) -> void {
-    if (auto *BV = Bound.dyn_cast<DIVariable *>()) {
+    if (auto *BV = dyn_cast_if_present<DIVariable *>(Bound)) {
       if (auto *VarDIE = getDIE(BV))
         addDIEEntry(DW_Subrange, Attr, *VarDIE);
-    } else if (auto *BE = Bound.dyn_cast<DIExpression *>()) {
+    } else if (auto *BE = dyn_cast_if_present<DIExpression *>(Bound)) {
       DIELoc *Loc = new (DIEValueAllocator) DIELoc;
       DIEDwarfExpression DwarfExpr(*Asm, getCU(), *Loc);
       DwarfExpr.setMemoryLocationKind();
       DwarfExpr.addExpression(BE);
       addBlock(DW_Subrange, Attr, DwarfExpr.finalize());
-    } else if (auto *BI = Bound.dyn_cast<ConstantInt *>()) {
+    } else if (auto *BI = dyn_cast_if_present<ConstantInt *>(Bound)) {
       if (Attr == dwarf::DW_AT_GNU_bias) {
         if (BI->getSExtValue() != 0)
           addUInt(DW_Subrange, Attr, dwarf::DW_FORM_sdata, BI->getSExtValue());