public boolean isRegASource(int reg) {
    return null != getSources().specForRegister(reg);
  }
Indicates whether the specified register is amongst the registers used as sources for this
instruction.

@param reg the register in question
@return true if the reg is a source
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/SsaInsn.java