`EXTRACTPS immb, xmm, r/m` is not included in this
change due to new ytab set 'yextractps'. This should simplify
code review.
4-operand instructions are a subject of upcoming changes that
make 4-th (and so on) operands explicit.
Related TODO note in asm6.go:
"dont't hide 4op, some version have xmm version".
Part of the mission to add missing amd64 SSE4 instructions to Go asm.