Start node stack push/pop setting IdT based on ParseNodeKind. (#2985)

I think there's more we can do here, but this seemed like a good
checkpoint to make sure the path I'm going down is roughly what you
expected. There's one actual edit in if expression structure to match
the increased enforcement.

toolchain/semantics/semantics_handle_if_expression.cpp

@@ -37,11 +37,10 @@ auto SemanticsHandleIfExpressionElse(SemanticsContext& context,
                                      ParseTree::Node else_node) -> bool {
   auto else_value_id = context.node_stack().Pop<SemanticsNodeId>();
   auto [then_node, then_end_block_id] =
-      context.node_stack().PopWithParseNode<SemanticsNodeBlockId>(
-          ParseNodeKind::IfExpressionThen);
+      context.node_stack().PopWithParseNode<ParseNodeKind::IfExpressionThen>();
   auto then_value_id = context.node_stack().Pop<SemanticsNodeId>();
   auto if_node =
-      context.node_stack().PopForSoloParseNode(ParseNodeKind::IfExpressionIf);
+      context.node_stack().PopForSoloParseNode<ParseNodeKind::IfExpressionIf>();
 
   // Convert the `else` value to the `then` value's type, and finish the `else`
   // block.
@@ -58,7 +57,7 @@ auto SemanticsHandleIfExpressionElse(SemanticsContext& context,
   context.AddCurrentCodeBlockToFunction();
 
   // Push the result value.
-  context.node_stack().Push(if_node, chosen_value_id);
+  context.node_stack().Push(else_node, chosen_value_id);
   return true;
 }