[gen] Annotation `X` read the wrong value in `mixed`/`fullmixed` modes

[ShaleXIONG]

A big thanks to @TiberiuBucur . we find a potential bug in diyone7 in mixed/fullmixed mode.

(* diyone7 -hexa -arch AArch64 -variant fullmixed -metadata false -oneloc PosRW X h0 Rfi *)
AArch64 CoRW1+posw0x.h0-rfix.h0w0
Variant=mixed
{
 uint32_t 0:X0=0; 0:X1=x;
}
 P0               ;
 LDR W0,[X1]      ;
 MOV W2,#257      ;
 Loop00:          ;
 LDXRH W3,[X1]    ;
 STXRH W4,W2,[X1] ;
 CBNZ W4,Loop00   ;

exists ([x]=0x101 /\ 0:X0=0x101 /\ 0:X3=0x100)

The post condition 0:X3=0x100 is incorrect which should be 0:X3=0. This reflect the scenario LDXRH and STXRH can be reordered before LDR. It is likely due to X use 0x101 - 1 to infer the LDXRH value. However in mixed mode, the value should be per-byte-based.