This changes the register class to GPRPair and adds the destination
register as a source with a tied operand constraint.
Parsing for the paired register is done with a custom parser that
checks for even register and converts it to its pair version. A
bit of care needs to be taken so that we only parse as a pair register
based on which instruction we're parsing and the mode in the subtarget.
This allows amocas.w to be parsed correcty in both modes.
I've added a FIXME to note that we should be creating pair registers
for Zdinx on RV32 to match the instructions CodeGen generates.
c053e9f0f4