[FunctionAttrs] Correct the safety analysis for inference of 'returned'

We skipped over ReturnInsts which didn't return an argument which would
lead us to incorrectly conclude that an argument returned by another
ReturnInst was 'returned'.

This reverts commit r275756.

This fixes PR28610.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276008 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed