Module PhiInsertion

Module to handle insertion of Phi instructions at the correct blocks. We insert a Phi instruction at the beginning of a block for a given location when that block is on the dominance frontier of a definition of the location and there is a use of that location reachable from that block without an intervening definition of the location. Within the approach outlined above, we treat a location slightly differently depending on whether or not it is a virtual variable. For a virtual variable, we will insert a Phi instruction on the dominance frontier if there is a use of any member location of that virtual variable that is reachable from the Phi instruction. For a location that is not a virtual variable, we insert a Phi instruction only if there is an exactly-overlapping use of the location reachable from the Phi instruction. This ensures that we insert a Phi instruction for a non-virtual variable only if doing so would allow dataflow analysis to get a more precise result than if we just used a Phi instruction for the virtual variable as a whole.