add DownCast for register id in executeFromSelfReg() #602
Description
To avoid manual casting to byte with .toByte.
Like for getVar and executeFromVar at http://github.com/ScorexFoundation/sigmastate-interpreter/blob/a598658d63ad822a99ef91ecc79da906ad779ffe/sigmastate/src/main/scala/sigmastate/lang/SigmaTyper.scala#L210-L210