re PR middle-end/54146 (Very slow compile with attribute((flatten)))

	PR middle-end/54146
	* gimpify.c (gimplify_body): Only verify_gimple_in_seq with
	checking enabled.
	* tree-ssa-loop-manip.c (add_exit_phis_var): Assert that var is
	a gimple_reg if checking is enabled.
	(find_uses_to_rename_stmt): Only look at non-virtual USE operands.
	* tree-into-ssa (compute_global_livein): Change the worklist
	type from an array to a VEC.

From-SVN: r190235