> I'm curious why incrementing in a loop can be unrolled and inspected to optimize to an addition, but doubling the number when both operands are equal can’t?

I expect because the former helps more in optimising real-world code than the latter. It’s not worth the LLVM developer's time to make the compiler better for programs that it won’t see in practice.

It’s not as if the compiler did nothing with that code, though. It replaced the multiplication by a left shift and removed the branch.