cmm: Always favour entry block during block deduplication.

Andreas Klebingerrequested to merge
wip/apk/cmm-cbe into master 
cmm: Always favour entry block during block deduplication.

We now always keep the first block in the CmmGraph. This way we avoid
the need to update the entry info table.

Failing to do so caused #27722.

Fixes #27722.

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