Use the index of the correct instruction to determine local names

Previously, we were using the index of the def instruction to determine
the names of the variables in the use instruction.

To make this change I needed to add a new map data structure to map
instructions to their index in the WALA IR instruction array.

See #23.
Disable Pi instructions in the IR

Saves us from having to deal with more instructions.

See #23.
Support Phi-node dependencies in the visualization

Accomplishes this by attaching Phi-nodes to their closest "real"
instruction.

See #23.