[RS6000] altivec style lvx/stvx addresses vs power10

[RS6000] altivec style lvx/stvx addresses vs power10

gcc.target/powerpc/fold-vec-st-pixel.c and other testcases fail on
power10, generating
	addi 9,5,12
	rldicr 9,9,0,59
	stxv 34,0(9)
rather than
	addi 5,5,12
	stvx 2,0,5
for an altivec lvx/stvx style address.

The problem starts with fwprop creating
(insn 9 4 0 2 (set (mem:V8HI (and:DI (plus:DI (reg/v/f:DI 121 [ vpp ])
                    (const_int 12 [0xc]))
                (const_int -16 [0xfffffffffffffff0])) [0 MEM <vector(8) short int> [(void *)_4 & -16B]+0 S16 A128])
        (reg/v:V8HI 120 [ vp1 ])) "pixel.c":6:10 1237 {vsx_movv8hi_64bit}
which is finally thrown out as invalid by lra.  lra of course does that
by reloading the entire address.

fwprop creates the invalid address due to rs6000_legitimate_address_p
trimming off the outer AND of altivec style addresses before applying
other predicates.  address_is_prefixed then allows the inner address.

Now at the time the AND stripping was added (git commit 850e8d3d56d),
rs6000_legitimate_address looked a lot simpler.  This patch allows
through just those addresses that were legitimate in those simpler
days.

	* config/rs6000/rs6000.c (rs6000_legitimate_address_p): Limit
	AND addressing to just lvx/stvx style addresses.