Aarch64 - Properly handle the zero register.

Currently this is a mess.

In compiler/CodeGen.Platform.h we have

-- stack pointer / zero reg
freeReg 31 = False

Which implies we use 31 as the register number for the zero reg.

However in compiler/GHC/CmmToAsm/AArch64/Regs.hs we have:

xzr = OpReg W64 (RegReal (RealRegSingle (-1)))
wzr = OpReg W32 (RegReal (RealRegSingle (-1)))

...

x31 = OpReg W64 (RegReal (RealRegSingle 31))

Encoding the zero register as -1 and defining a general purpose register 31 which doesn't actually exist on the platform.

We also include register 31 in

allMachRegNos   :: [RegNo]
allMachRegNos   = [0..31] ++ [32..63]

In the pretty printer however we define the register 31 unconditionally as the stack register.

    ppr_reg_no :: Width -> Int -> SDoc
    ppr_reg_no w 31
         | w == W64 = text "sp"
         | w == W32 = text "wsp"

The fact that this doesn't currently cause bugs is only due to two wrongs making a right in this case.

The definition freeReg 31 = False aimed at the zero register means that the non-existant GP register x31 never ends up being allocated by the register allocator.