[NFC][TableGen] Code cleanup in CodeGenRegister (llvm#137994)

- Use range for loops.
- Wrap complex LLVM_DEBUG() macros in {} to have clang-format format the
contents similar to regular code.
- Extract repeated code snippets for debug dumping into helper lambda.
- Add `BitsInit::getBits()` to get all contained bits.

Author: jurahul

llvm/include/llvm/TableGen/Record.h

@@ -632,10 +632,11 @@ class BitsInit final : public TypedInit,
 
   const Init *resolveReferences(Resolver &R) const override;
 
-  const Init *getBit(unsigned Bit) const override {
-    assert(Bit < NumBits && "Bit index out of range!");
-    return getTrailingObjects<const Init *>()[Bit];
+  ArrayRef<const Init *> getBits() const {
+    return ArrayRef(getTrailingObjects<const Init *>(), NumBits);
   }
+
+  const Init *getBit(unsigned Bit) const override { return getBits()[Bit]; }
 };
 
 /// '7' - Represent an initialization by a literal integer value.
@@ -781,10 +782,12 @@ class ListInit final : public TypedInit,
 
   void Profile(FoldingSetNodeID &ID) const;
 
-  const Init *getElement(unsigned i) const {
-    assert(i < NumValues && "List element index out of range!");
-    return getTrailingObjects<const Init *>()[i];
+  ArrayRef<const Init *> getValues() const {
+    return ArrayRef(getTrailingObjects<const Init *>(), NumValues);
   }
+
+  const Init *getElement(unsigned Index) const { return getValues()[Index]; }
+
   const RecTy *getElementType() const {
     return cast<ListRecTy>(getType())->getElementType();
   }
@@ -804,12 +807,8 @@ class ListInit final : public TypedInit,
   bool isConcrete() const override;
   std::string getAsString() const override;
 
-  ArrayRef<const Init *> getValues() const {
-    return ArrayRef(getTrailingObjects<const Init *>(), NumValues);
-  }
-
-  const_iterator begin() const { return getTrailingObjects<const Init *>(); }
-  const_iterator end  () const { return begin() + NumValues; }
+  const_iterator begin() const { return getValues().begin(); }
+  const_iterator end() const { return getValues().end(); }
 
   size_t         size () const { return NumValues;  }
   bool           empty() const { return NumValues == 0; }