Generate SSA code from returns (#5172)

Modify return pseudo-instructions to have pairs of registers: virtual and real. This allows us to constrain the virtual registers to the real ones specified by the abi, instead of directly emitting moves to those real registers.

cranelift/codegen/src/isa/x64/inst.isle

@@ -333,7 +333,7 @@
         (args VecArgPair))
 
        ;; Return.
-       (Ret (rets VecReg))
+       (Ret (rets VecRetPair))
 
        ;; Jump to a known target: jmp simm32.
        (JmpKnown (dst MachLabel))