Explain order of operations in _useUnorderedNonce

[PaulRBerg]

In SignatureTransfer._userUnorderedNonce, there is this line:

permit2/src/SignatureTransfer.sol

Line 153 in bbbc92f

uint256 flipped = nonceBitmap[from][wordPos] ^= bit;

Based on the discussion in #120, I understand that the double assignment was implemented for gas efficiency purposes (that is, for the happy execution paths). However, this is an uncommon design choice, so it would be helpful to explain the order of operations, i.e. that the storage assignment is performed first, and then the resulting value is assigned to the flipped variable (generally speaking, it's not obvious that y = z is performed first in x = y = z).

Also, the NatSpec comments mention the check before the storage update, although in practice the reverse order of operations applies:

permit2/src/SignatureTransfer.sol

Line 147 in bbbc92f

/// @notice Checks whether a nonce is taken and sets the bit at the bit position in the bitmap at the word position

Side note: this design deviates from the Checks-Effects-Interactions pattern, which is not a bad thing in and of itself, but this is worth mentioning FYI.