Introduce environment variable ID for LoopEnvironment and keep index internal to LoopEnvironmentBuilder & User

public:
- rename method getEnvIndicesOfLiveInVars -> getEnvIDsOfLiveInVars
- rename method getEnvIndicesOfLiveOutVars -> getEnvIDsOfLiveOutVars
- rename method indexOfExitBlockTaken -> getExitBlockID
- rename method producerAt -> getProducer
private:
- rename method indexOfProducer -> getProducerID (this method doesn't get invoked)

public:
- addVariableToEnvironment: use ID instead of index, update both maps after adding a new variable
- hasVariableBeenReduced: change to use id and do the map check at beginning
- getEnvironmentVariable: change to use id and do the map check at beginning
- getAccumulatedReducedEnvironmentVariable: change to use id and do the map check at beginning
- getReducedEnvironmentVariable: change to use id and do the map check at beginning
- reduceLiveOutVariables: assume parameters are all ID based, replace with index internally if necessary
- create method getIndexOfEnvironmentVariable given ID
private:
- add two unordered_maps envIDToIndex & indexToEnvID
- initializeBuilder: generate envID to index map and reverse map, store envTypes based on selected envIDs

public:
- Constructor: take unordered_map envIDToIndex as a parameter passed from the builder
- rename method addLiveInIndex -> addLiveIn
- rename method addLiveOutIndex -> addLiveOut
- delete methods getEnvIndicesOfLiveInVars & getEnvIndicesOfLiveOutVars
- add methods getEnvIDsOfLiveInVars & getEnvIDsOfLiveOutVars
- createEnvironmentVariablePointer: change to use id and do the map check at beginning
- createReducableEnvPtr: change to use id and do the map check at beginning
private:
- add unordered_map envIDToIndex
- add liveInIDs & liveOutIDs
- delete liveInInds & liveOutInds

public:
- create method getIndexOfEnvironmentVariable, which is a wrapper of this->envBuilder->getIndexOfEnvironmentVariable

arcana-lab / noelle