[MIR] Reintroduce the unit temporary

[MIR] Reintroduce the unit temporary

An attempt to make loop body destination be optional, author implemented a pretty self contained
change and deemed it to be (much) uglier than the alternative of just keeping the unit temporary.
Having the temporary created lazily also has a nice property of not figuring in the MIR of
functions which do not use loops of any sort.