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_class.cpp

@@ -55,7 +55,7 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId parse_node)
 
   // Add the class declaration.
   auto class_decl = SemIR::ClassDecl{SemIR::ClassId::Invalid, decl_block_id};
-  auto class_decl_id = context.AddInst({parse_node, class_decl});
+  auto class_decl_id = context.AddPlaceholderInst({parse_node, class_decl});
 
   // Check whether this is a redeclaration.
   auto existing_id =
@@ -110,7 +110,7 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId parse_node)
   }
 
   // Write the class ID into the ClassDecl.
-  context.insts().Set(class_decl_id, class_decl);
+  context.ReplaceInstBeforeConstantUse(class_decl_id, {parse_node, class_decl});
 
   return {class_decl.class_id, class_decl_id};
 }
@@ -304,9 +304,9 @@ auto HandleBaseDecl(Context& context, Parse::BaseDeclId parse_node) -> bool {
 
   // Add a corresponding field to the object representation of the class.
   // TODO: Consider whether we want to use `partial T` here.
-  context.args_type_info_stack().AddInst(
+  context.args_type_info_stack().AddInstId(context.AddInstInNoBlock(
       {parse_node,
-       SemIR::StructTypeField{SemIR::NameId::Base, base_info.type_id}});
+       SemIR::StructTypeField{SemIR::NameId::Base, base_info.type_id}}));
 
   // Bind the name `base` in the class to the base field.
   context.decl_name_stack().AddNameToLookup(