public int borrowSpareRegister(int category) {
    int result = spareRegisterBase + borrowedSpareRegisters;

    borrowedSpareRegisters += category;
    registerCount = Math.max(registerCount, result + category);

    return result;
  }
Borrows a register to use as a temp. Used in the phi removal process. Call
returnSpareRegisters() when done.

@param category width (1 or 2) of the register
@return register number to use
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaMethod.java