I always found the phi instruction pretty unintuitive, because
1. I thought that due to the SSA form the register uniquely determines the basic block and thus the phi instruction should not need the information of a "predecessor block".
2. I remember to have seen valid LL files where the predecessor block in a phi instruction was not an immediate predecessor.
I thought that it would be more intuitive to have input and output parameters in basic blocks like so:
start: param (i32 %0, i1 %1)
br i1 %1, label start (%2, %3), label stop (%4)
stop: param (i16 %5)
Has anybody thought about such a structure, yet?