compute: fix case on type assertion

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/26527.

Fixes a change that broke the regex capture groups result functionality when I introduced this switch statement yesterday.

The type of the value here is actually compute.MatchOnly, and not a pointer. I forgot I need to write tests for these kinds of things because compiler can't help me 😠 . I'll add a test in a follow up, just want to get this fix in first.