XCHG is mis-decoded as NOP

Created by: dvyukov

Decoding this byte sequence:

echo -en "\x66\x41\x90" > /tmp/bin | objdump -mi386 -Mx86-64 -b binary -D /tmp/bin
   0:	66 41 90             	xchg   %ax,%r8w

objdump correctly says it's an XCHG. While DynamoRIO decodes it as just nop. There are several 90 encodings that are XCHG rather than NOP:

90+rw XCHG AX, r16 Exchange r16 with AX.
90+rw XCHG r16, AX Exchange AX with r16.
90+rd XCHG EAX, r32 Exchange r32 with EAX.

On commit 6e887383.