[ThinLTO] Fix unreachable code when parsing summary entries.

Summary:
Early returns were causing some code to be skipped. This was missed
since the summary entries are typically at the end of the llvm assembly
file.

Fixes PR41663.

Reviewers: RKSimon, wristow

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61355

llvm-svn: 359697