Label each BasicBlock with the value it *must* return.

Most frontends that generate bitcode unify all of the return statements of a function and return a phi node that has a return value for each branch. This pass (labelBlockReturns) pushes those returns backwards through the control flow graph as labels on basic blocks. The function blockReturn gives the return value for a block, if there is a value that must be returned by that block.

The algorithm starts from the return instruction. Non-phi values are propagated backwards to all reachable blocks. Phi values are split and the algorithm propagates each phi incoming value back to the block it came from. A value can be propagated from a block BB to its predecessor block PB if (and only if) BB postdominates PB. Intuitively, the algorithm propagates a return value to a predecessor block if that predecessor block *must* return that value (hence postdominance).