split_USE and split_Rematerialize should attempt to reuse copies

split_USE and split_Rematerialize can produce multiple copies of the same value in one block if there are multiple users of that value. Often post allocate copy removal will be able to remove these redundant copies but because of the vagaries of register selection it might not be able to in all cases. This will cause code where a value is reloaded or rematerialized and then immediately use and killed, only to be reloaded again. A partial fix for this is to modify split_USE and split_Rematerialize to try to reuse copies within a block when the def is outside the block. If the def is in the same block as the use then it's likely that a new copy if really needed so it's not done in that case.