Shift 16-bit left works for all registers

Shift 16-bit register left now works for registers B-D as well, and has working unit tests.

The RPT counter register has also been expanded to 12 bits, so in hardware we’ll build it with two 6-bit chips. This isn’t needed for the shift instructions, but will add flexibility for future microcode.

Next step will be implementing the 8-bit register shift left instructions.