Optimise variational action2 trivial cases

[JGRennison]

Drop useless binary operator operations with constant 0 for:

• shift left/right
• or
• xor
• rotate

Optimise trivial right shifts of variables by a constant value. Adjust the shift right and mask of a variable where possible, in the same way as is currently done for left shifts of inherently right shifted variables.

The practical upshot of this is that extremely common patterns like: getbits(extra_callback_info2, 0, 8) now result in the obvious representation:

• ADD/RST variable 18, shift: 0, mask: FF

Instead of currently:

• ADD/RST variable 18, shift: 0, mask: FFFFFFFF
• SHR variable 1A, shift: 0, mask: 0
• AND variable 1A, shift: 0, mask: FF

Using one instead of three ops allows direct references in more cases instead of needing to spill to a temporary register and use RST.

[glx22]

Regression fails, but the output clearly shows the optimisation.

[FLHerne]

Nice improvement.

This looks correct to me - just need to update regression/expected/* with the new results.