public int getNextReg() {
    return reg + getCategory();
  }
Gets the next available register number after the one in this instance. This is equal to the
register number plus the width (category) of the type used. Among other things, this may also
be used to determine the minimum required register count implied by this instance.

@return {@code >= 0;} the required registers size
android/android_framework/dex-maker/src/main/java/com/android/dx/rop/code/RegisterSpec.java