[AArch64][GlobalISel] Check if G_SELECT has been optimized when folding binops

`TryFoldBinOpIntoSelect` didn't have a check for `Optimized`, meaning you could
end up folding twice. (e.g. a select with a G_ADD on the true side, and a G_SUB
on the false side)

Add in the missing `if` and a test.