Model type expressions as regions (#4698)

This is a precondition for enabling the new pattern-matching subsystem
to support binding patterns that have `if` expressions in the type
position.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_loop_statement.cpp

@@ -21,7 +21,7 @@ auto HandleParseNode(Context& context, Parse::WhileConditionStartId node_id)
 
   // Start emitting the loop header block.
   context.inst_block_stack().Push(loop_header_id);
-  context.AddCurrentCodeBlockToFunction();
+  context.AddToRegion(loop_header_id, node_id);
 
   context.node_stack().Push(node_id, loop_header_id);
   return true;
@@ -42,7 +42,7 @@ auto HandleParseNode(Context& context, Parse::WhileConditionId node_id)
 
   // Start emitting the loop body.
   context.inst_block_stack().Push(loop_body_id);
-  context.AddCurrentCodeBlockToFunction();
+  context.AddToRegion(loop_body_id, node_id);
   context.break_continue_stack().push_back(
       {.break_target = loop_exit_id, .continue_target = loop_header_id});
 
@@ -64,7 +64,7 @@ auto HandleParseNode(Context& context, Parse::WhileStatementId node_id)
 
   // Start emitting the loop exit block.
   context.inst_block_stack().Push(loop_exit_id);
-  context.AddCurrentCodeBlockToFunction();
+  context.AddToRegion(loop_exit_id, node_id);
   return true;
 }