Fix regression for mapIt (#8567)
Fix regression for mapIt (#8567)

Don't try to be too smart and limit the use of `evalOnce` where strictly
needed as not every value can be assigned with a `let`.

Fixes #8566
dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim