Separate the concept of 16-bit/32-bit operand size controlled by 0x66 prefix...

Separate the concept of 16-bit/32-bit operand size controlled by 0x66 prefix and the current mode from the concept of SSE instructions using 0x66 prefix as part of their encoding without being affected by the mode.

This should allow SSE instructions to be encoded correctly in 16-bit mode which r198586 probably broke.

llvm-svn: 199193