The shl and shr instructions are often documented to take both a Vx and a
Vy argument.  It’s a bit unclear what the behaviour here is meant to be.
The original CHIP-8 implementation shifted Vy by 1 and stored the result
in Vx.  Some GitHub projects shift the value in Vx by the value in Vy.
Ahoy takes the approach of SCHIP where Vx is shifted by 1 and Vy is
ignored.  The Ahoy assembler does not recognize a second Vy argument to
either instruction.