small improvement for split trades. when calculating destAmount

[ilanDoron]

bytecode size diff after this change: 23376 before this change: 23369

gas for split before: 2e: 541075 gas used, type: SPLIT fee: 123 num reserves: 3 e2t: 531420 gas used, type: SPLIT fee: 123 num reserves: 3 t2t: 921553 gas used, type: SPLIT fee: 123 num reserves: 3

after: t2e: 540689 gas used, type: SPLIT fee: 123 num reserves: 3 e2t: 531074 gas used, type: SPLIT fee: 123 num reserves: 3 t2t: 920821 gas used, type: SPLIT fee: 123 num reserves: 3

non split before: t2e: 478351 gas used, type: NO HINT fee: 123 num reserves: 3 e2t: 519036 gas used, type: NO HINT fee: 123 num reserves: 3 t2t: 878734 gas used, type: NO HINT fee: 123 num reserves: 3

after: t2e: 478359 gas used, type: NO HINT fee: 123 num reserves: 3 e2t: 519044 gas used, type: NO HINT fee: 123 num reserves: 3 t2t: 878750 gas used, type: NO HINT fee: 123 num reserves: 3