AArch64: save two words in itable lookup stub

vtable stubs are hot in some apps. Looking to asm codes we see it can make it a bit shorter.

$ java -XX:+PrintAdapterHandlers
Decoding VtableStub itbl[3]@2
    ------------------------- VtableStubs::create_itable_stub
 1 ldp x16, x12, [x9]
 2 ldr w10, [x1, #8]
 3 eor x10, x10, #0x800000000
    ......................... lookup_interface_method(1)
 4 ldr w11, [x10, #16]
 5 add x11, x10, x11, uxtx #3
 6 add x11, x11, #0x1d0
 7 ldr x15, [x11]
 8 cmp x12, x15
 9 b.eq 0x0000ffff710ff2f8
10 cbz x15, 0x0000ffff710ff338
11 add x11, x11, #0x10 ------->
12 ldr x15, [x11] ------------------> ldr x15, [x11, #16]!
13 cmp x12, x15
14 b.ne 0x0000ffff710ff2e4
    ......................... lookup_interface_method(2)
15 ldr w11, [x10, #16]
16 add x11, x10, x11, uxtx #3
17 add x11, x11, #0x1d0
18 add x10, x10, #0x18
19 ldr x12, [x11]
20 cmp x16, x12
21 b.eq 0x0000ffff710ff328
22 cbz x12, 0x0000ffff710ff338
23 add x11, x11, #0x10 ------>
24 ldr x12, [x11] -----------------> ldr x12, [x11, #16]!
25 cmp x16, x12
26 b.ne 0x0000ffff710ff314
27 ldr w11, [x11, #8]
28 ldr x12, [x10, w11, uxtw]
    ......................... VtableStubs::create_itable_stub (continue)
29 ldr x8, [x12, #56]
30 br x8
31 adrp x8, Stub::<unknown>
32 add x8, x8, #0xe80
33 br x8
34 .inst 0x00000000 ; undefined