Defining constraint for registers tuple

[Kirill Yukhin]

Hi guys,
I'm working on implementation of `mulx` (which is part of BMI2). One
of improvements compared generic `mul` is that it allows to specify
destination registers.
For `mul` we have `A` constraint, which stands for AX:DX pair.
So, is there a possibility to relax such cinstraint and allow any pair
of registers as destination?

Thanks, K