Move `INTEGER`/`REAL`/other accessors out of loops

> We are always so picky about moving `INTEGER` out of loops, but apparently here we did not care yet.

_Originally posted by @ben-schwen in https://github.com/Rdatatable/data.table/pull/7598#discussion_r2702792455_

It might be possible to write a linter that detects some cases where the `expression` in `accessor(expression)` does not depend on the current loop iteration.