Rename PROP_trees to PROP_gimple

PROP_trees actually means GIMPLE IL, rather than GENERIC, so better
not to confuse users.

2021-01-27  Jakub Jelinek  <jakub@redhat.com>

	* tree-pass.h (PROP_trees): Rename to ...
	(PROP_gimple): ... this.
	* cfgexpand.c (pass_data_expand): Replace PROP_trees with PROP_gimple.
	* passes.c (execute_function_dump, execute_function_todo,
	execute_one_ipa_transform_pass, execute_one_pass): Likewise.
	* varpool.c (ctor_for_folding): Likewise.