public RegisterSpec get(RegisterSpec spec) {
    return get(spec.getReg());
  }
Gets the element with the same register number as the given spec, if any. This is just a
convenient shorthand for {@code get(spec.getReg())}.

@param spec {@code non-null;} spec with the desired register number
@return {@code null-ok;} the element with the matching register number or {@code null} if there
    is none
android/android_framework/dex-maker/src/main/java/com/android/dx/rop/code/RegisterSpecSet.java