[gen] Add pair-cell offset annotation for AArch64.

[ShaleXIONG]

Expose a non-mixed AArch64 offset annotation to generate ordinary accesses to the second cell of a pair-backed location. The surface spelling follows the current natural type size.

• with the default 32-bit int, the added annotation is w4
• with -type int64_t, it is q8.

We also remove the synthetic ADD after AArch64 pair loads but check both register values in the final condition.
Given the new annotation and change, we can generate test, e.g. diyone7 -metadata false -arch AArch64 -oneloc Rfe PaIQ Fre w4 PosWW

AArch64 WW+R+posw4p+PaIQ
{
 int x[2]={0,0};
 uint32_t 0:X0=0; 0:X1=x;
 1:X1=x;
}
 P0                                                      | P1                ;
 MOV W0,#2                                               | LDIAPP W0,W2,[X1] ;
 STR W0,[X1,#4] (* w4 annotation to access second cell *)|                   ;
 MOV W2,#1                                               |                   ;
 STR W2,[X1]                                             |                   ;

exists (x={1,2} /\ 1:X0=1 /\ 1:X2=0)