Strength reduce function 'update_map' can be refactored

[LucasSte]

@xermicus had a good idea to improve the update_map function in strength reduce, as discussed here. We should try that when there is time to do so.

https://github.com/hyperledger-labs/solang/blob/2b7fc1c0416720ba26c5eabcd97901988866aef3/src/codegen/strength_reduce/reaching_values.rs#L98

[seanyoung]

Anyone can pick up this issue when they feel like it.