public int predBlockIndexForSourcesIndex(int sourcesIndex) {
    return operands.get(sourcesIndex).blockIndex;
  }
Gets the index of the pred block associated with the RegisterSpec at the particular
getSources() index.

@param sourcesIndex index of source in getSources()
@return block index
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/PhiInsn.java