target-i386: fix SIB decoding with index = 4

A SIB byte with an index of 4 means "no scaled index", even if the scale
value is not 0. In 64-bit mode, if REX.X is used, an index of 4 selects
%r12. This is correctly handled by the computation of the index variable,
which includes the index bits, and also the REX.X prefix:

    index = ((code >> 3) & 7) | REX_X(s);

Thanks to Avi Kivity, Jamie Lokier and Malc for the analysis of the
problem and the initial patch.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-i386/translate.c

@@ -2047,8 +2047,8 @@ static void gen_lea_modrm(DisasContext *s, int modrm, int *reg_ptr, int *offset_
                 gen_op_movl_A0_im(disp);
             }
         }
-        /* XXX: index == 4 is always invalid */
-        if (havesib && (index != 4 || scale != 0)) {
+        /* index == 4 means no index */
+        if (havesib && (index != 4)) {
 #ifdef TARGET_X86_64
             if (s->aflag == 2) {
                 gen_op_addq_A0_reg_sN(scale, index);