This was made possible by the recent change to codeGen to attach the
live GlobalRegs to every CmmJump, and we'll be relying on it quite
heavily in the new code generator too.

What this means essentially is that when we see

  x = R1

the register allocator will automatically assign x to R1 and generate
no code at all (also known as "coalescing"). It wasn't possible before
because the register allocator had to assume that R1 was always live,
because it didn't have access to accurate liveness information.