A panic in CmmBuildInfoTables.bundle shouldn't be a panic (#9329)

Summary:
This code needs more comments, but I believe this is safe.  By
definition I can't have broken anything that was working by turning a
panic into a non-panic anyway.

Test Plan: validate

Reviewers: hvr, simonpj, austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D105

GHC Trac Issues: #9329