Handle arithmetic on eliminated address indices [PR116413]

This patch fixes gcc.c-torture/compile/opout.c for m68k with LRA
enabled.  The test has:

...
z (a, b)
{
  return (int) &a + (int) &b + (int) x + (int) z;
}

so it adds the address of two incoming arguments.  This ends up
being treated as an LEA in which the "index" is the incoming
argument pointer, which the LEA multiplies by 2.  The incoming
argument pointer is then eliminated, leading to:

(plus:SI (plus:SI (ashift:SI (plus:SI (reg/f:SI 24 %argptr)
                (const_int -4 [0xfffffffffffffffc]))
            (const_int 1 [0x1]))
        (reg/f:SI 41 [ _6 ]))
    (const_int 20 [0x14]))

In the address_info scheme, the innermost plus has to be treated
as the index "term", since that's the thing that's subject to
index_reg_class.

gcc/
	PR middle-end/116413
	* rtl.h (address_info): Update commentary.
	* rtlanal.cc (valid_base_or_index_term_p): New function, split
	out from...
	(get_base_term, get_index_term): ...here.  Handle elimination PLUSes.