Noticed by @simonmar in !1362:

    If the srtEntry is Nothing, then it should be safe to omit
    references to this SRT from other SRTs, even if it is a static
    function.

When updating SRT map we don't omit references to static functions (see
Note [Invalid optimisation: shortcutting]), but there's no reason to add
an SRT entry for a static function if the function is not CAFFY.

(Previously we'd add SRT entries for static functions even when they're
not CAFFY)

Using 9151b99e I checked sizes of all SRTs when building GHC and
containers:

- GHC: 583736 (HEAD), 581695 (this patch). 2041 less SRT entries.
- containers: 2457 (HEAD), 2381 (this patch). 76 less SRT entries.