Refactor the code that determines whether to show the "Merge Immediately" dropdown

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The following discussion from !14894 (merged) should be addressed:

• @nfriend started a discussion: (+10 comments)

  @dosuken123 This is the CE logic that determines whether or not the merge button dropdown is shown. This logic is identical to what it was before: https://gitlab.com/gitlab-org/gitlab-ee/commit/35ae9d8a7449ebe4b5c1878825bcb931a82e7a59?view=parallel#687534b2e73644a518a1110160738f4d84731d09_97_97.

Specifically, this piece of code should be simplified. In it's current form, it's hard to understand what's going on without building a table of all possible outcomes.