public int getPrimarySuccessorIndex() {
    return primarySuccessorIndex;
  }
Gets the primary successor index. This is the index into the successors list where the primary
may be found or {@code -1} if there are successors but no primary successor. This may return
something other than {@code -1} in the case of an instruction with no successors at all
(primary or otherwise).

@return {@code >= -1;} the primary successor index
android/android_framework/dex-maker/src/main/java/com/android/dx/cf/code/RopperMachine.java