public void addPhiInsnForReg(int reg) {
    insns.add(0, new PhiInsn(reg, this));
  }
Adds a phi insn to the beginning of this block. The result type of the phi will be set to void,
to indicate that it's currently unknown.

@param reg {@code >=0;} result reg
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaBasicBlock.java