private int findNextUnreservedRopReg(int startReg, int regCategory) {
    return findNextUnreservedRopReg(startReg, regCategory, getAlignment(regCategory));
  }
Finds unreserved rop registers with a specific category.

@param startReg {@code >= 0;} a rop register to start the search at
@param regCategory {@code > 0;} category of the searched registers.
@return {@code >= 0;} start of available registers.
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/back/FirstFitLocalCombiningAllocator.java