You can see all optimization stages on godbolt.org. Above the assembly view, click Add new and select Opt pipeline. (It's available if you've picked a recent clang compiler in the compiler dropdown.) Then you can click through the optimization list and find which one made the change you're interested in.
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You can see all optimization stages on godbolt.org. Above the assembly view, click Add new and select Opt pipeline. (It's available if you've picked a recent clang compiler in the compiler dropdown.) Then you can click through the list of passes in the optimization panel, and find which pass made the change you're interested in. The passes that applied have green text in the list.