Ensure we evaluate instructions created in uncommon ways. (#3598)

Instructions created by splices during conversion are now evaluated, as
are instructions created in cases where we first create a placeholder
instruction and later replace it by a different instruction.

This also removes the ability to set a parse node and instruction
independently after creating an `InstId`, which could lead to them
accidentally not matching.

toolchain/check/handle_struct.cpp

@@ -40,9 +40,9 @@ auto HandleStructFieldValue(Context& context,
   auto [name_node, name_id] = context.node_stack().PopNameWithParseNode();
 
   // Store the name for the type.
-  context.args_type_info_stack().AddInst(
+  context.args_type_info_stack().AddInstId(context.AddInstInNoBlock(
       {name_node, SemIR::StructTypeField{
-                      name_id, context.insts().Get(value_inst_id).type_id()}});
+                      name_id, context.insts().Get(value_inst_id).type_id()}}));
 
   // Push the value back on the stack as an argument.
   context.node_stack().Push(parse_node, value_inst_id);