[SystemZ] Add NRK, ORK and XRK

The atomic tests assume the two-operand forms, so I've restricted them to z10.

Running and-01.ll, or-01.ll and xor-01.ll for z196 as well as z10 shows why
using convertToThreeAddress() is better than exposing the three-operand forms
first and then converting back to two operands where possible (which is what
I'd originally tried).  Using the three-operand form first stops us from
taking advantage of NG, OG and XG for spills.