KVM: x86 emulator: simplify two-byte opcode check

Two-byte opcode always start with 0x0F and the decode flags
of opcode 0xF0 is always 0, so remove dup check.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>