@Override
          public void visitPhiInsn(PhiInsn phi) {
            definitionList[phi.getResult().getReg()] = phi;
          }
Returns the insn that defines the given register

@param reg register in question
@return insn (actual instance from code) that defined this reg or null if reg is not defined.
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaMethod.java