Clean up interface of node block stack after recent changes. (#3229)

- `Peek()` no longer returns a useful value, because we often don't
allocate a `NodeBlockId` until we finish building the block. Remove it
and update both its callers.
- Rename `PeekForAdd()` to `PeekOrAdd()` since it's no longer used to
get a block ID to add elements into.
- `PushForAdd()` was unused. Remove it.
- Add `NodeBlockStack::AddNode` to combine the operations of adding a
node and inserting it into a block.

toolchain/check/node_block_stack.cpp

@@ -22,13 +22,12 @@ auto NodeBlockStack::Push(SemIR::NodeBlockId id) -> void {
   ++size_;
 }
 
-auto NodeBlockStack::PeekForAdd(int depth) -> SemIR::NodeBlockId {
+auto NodeBlockStack::PeekOrAdd(int depth) -> SemIR::NodeBlockId {
   CARBON_CHECK(size() > depth) << "no such block";
   int index = size() - depth - 1;
   auto& slot = stack_[index];
   if (!slot.id.is_valid()) {
     slot.id = semantics_ir_->AddNodeBlockId();
-    CARBON_VLOG() << name_ << " Add " << index << ": " << slot.id << "\n";
   }
   return slot.id;
 }