Add an assert to leaf_function_p to ensure it is not called from a

Add an assert to leaf_function_p to ensure it is not called from a
prolog or epilog sequence (which would incorrectly return true in a
non-leaf function).  There are several targets which still call
leaf_function_p, and while most appear safe or appear aware of the
issue, it is likely not all such calls are safe.  This check enables
any such latent bugs to be found.

    gcc/
	* final.c (leaf_function_p): Check we are not in a sequence.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@248240 138bc75d-0d04-0410-961f-82ee72b054a4