phi nodes must be in block header

[mnikander]

• check that phi nodes appear only at the start of the block, with no other instructions before it (except the block label)
• decision documented
• ensure that when there are multiple phi nodes in a block, they do not bind the result of another phi node
• decision documented

This simplifies the static analysis, since phi nodes must only consider the incoming edges, and never the instructions in the block
Open question: should a phi node be allowed to mention a variable defined by a phi-node in the same block?

x = phi ...
y = phi x ...

I think the answer should be NO, since there is no incoming edge, with which a phi-bound value in the current block can be associated. That breaks the "if I came form here then take this value, if I came from there then take the other value" semantics of the phi node.