function: Do the CLEANUP_EXPENSIVE after shrink-wrapping, not before

function: Do the CLEANUP_EXPENSIVE after shrink-wrapping, not before

We should do CLEANUP_EXPENSIVE after shrink-wrapping, because shrink-
wrapping creates constructs that CLEANUP_EXPENSIVE can optimise, and
nothing runs CLEANUP_EXPENSIVE later.

	* function.c (rest_of_handle_thread_prologue_and_epilogue): Call
	cleanup_cfg with CLEANUP_EXPENSIVE after shrink-wrapping instead
	of before.  Add a comment.

From-SVN: r236372