Refactor InstBlockStack to use ArrayStack. (#4104)

The use of ArrayStack here is intended to simplify the logic, and also
make better use of the inst heap allocations. Prior changes #4101 and
#4103 removed the less related logic from InstBlockStack, although #4103
is the actual part that blocked using ArrayStack.

BTW, note the PrintForStackDump implementation was incorrect because it
didn't apply size_. This simplification fixes the issue.

common/array_stack.h

@@ -47,6 +47,15 @@ class ArrayStack {
     return llvm::ArrayRef(values_).slice(array_offsets_.back());
   }
 
+  // Returns the array at a specific index.
+  auto PeekArrayAt(int index) const -> llvm::ArrayRef<ValueT> {
+    auto ref = llvm::ArrayRef(values_).slice(array_offsets_[index]);
+    if (index + 1 < static_cast<int>(array_offsets_.size())) {
+      ref = ref.take_front(array_offsets_[index + 1] - array_offsets_[index]);
+    }
+    return ref;
+  }
+
   // Returns the full set of values on the stack, regardless of whether any
   // arrays are pushed.
   auto PeekAllValues() const -> llvm::ArrayRef<ValueT> { return values_; }
@@ -58,6 +67,13 @@ class ArrayStack {
     values_.push_back(value);
   }
 
+  // Adds multiple values to the top array on the stack.
+  auto AppendToTop(llvm::ArrayRef<ValueT> values) -> void {
+    CARBON_CHECK(!array_offsets_.empty())
+        << "Must call PushArray before PushValues.";
+    values_.append(values.begin(), values.end());
+  }
+
   // Returns the current number of values in all arrays.
   auto all_values_size() const -> size_t { return values_.size(); }