x86/x64: Fix instruction length decoder.

Thanks to Peter Cawley.

src/lj_asm_x86.h

@@ -2836,9 +2836,9 @@ static uint32_t asm_x86_inslen(const uint8_t* p)
     case 4: result -= (prefixes & 2);  /* fallthrough */
     case 5: return result + (x & 15);
     case 6:  /* Group 3. */
-      if (p[1] & 0x38) return result + 2;
-      if ((prefixes & 2) && (x == 0x66)) return result + 4;
-      return result + (x & 15);
+      if (p[1] & 0x38) x = 2;
+      else if ((prefixes & 2) && (x == 0x66)) x = 4;
+      goto mrm;
     case 7: /* VEX c4/c5. */
       if (LJ_32 && p[1] < 0xc0) {
 	x = 2;