Decode additional segment prefixes

This is mainly needed to handle the new control flow enforcement
extensions, making 3E a "notrack" prefix for indirect calls and jumps.

This is not (yet) modeled, and requires additional information on the
order of the prefixes, as 3E_66 (16-bit in ds segment) has a different
meaning than 66_3E (16-bit notrack). Before implementing this, an
analysis of the performance impact when decoding more prefix information
is probably required to avoid degrading overall performance for very few
and (as of now) seldomly used corner cases.

decode.h

@@ -75,6 +75,8 @@ enum PrefixSet
     PREFIX_SEG_FS = 1 << 0,
     PREFIX_SEG_GS = 1 << 1,
     PREFIX_SEG_CS = 1 << 12,
+    PREFIX_SEG_DS = 1 << 17,
+    PREFIX_SEG_ES = 1 << 18,
     PREFIX_OPSZ = 1 << 2,
     PREFIX_ADDRSZ = 1 << 3,
     PREFIX_LOCK = 1 << 4,