Do not store up to three registers for an instruction.
Instead only store the explicit register operand and whether the operands are permuted and/or the result is to be placed in the explicit register operand or st0.

Whether the operands are permuted can be deduced by testing op1_idx != 0.

Let a flag and the emitter handle printing the pop variant.

Let a flag and the emitter handle printing the pop variant.

Let a flag and the emitter handle printing the pop variant.

Let the emitter handle printing the r by checking the assigned registers.

out cannot be null.

Simulating it as normal binop does only work in one special case.
It lacks the reverse form and selecting the second operand register.
So in many cases invalid code was generated silently.

It is just the sum of family and extended family.