[llvm] [CSSPGO] Return error_code for missing probe profile (PR #102085)

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WenleiHe wrote:

> Given that pre-link sample-profile is before inline

the profile annotation part (`computeAndPropagateWeights`) is actually done after the inlining in sample loader itself, so technically speaking, the part you are changing here is after inlining for pre-link.


https://github.com/llvm/llvm-project/pull/102085