Uses assume to check the length against a constant upper bound. The
inlined result then informs the optimizer of the sound value range.
This was tried with unreachable_unchecked before which introduces a
branch. This has the advantage of not being executed in sound code but
complicates basic blocks. It resulted in ~2% increased compile time in
some worst cases.
Add a codegen test for the assumption, testing the issue from #67186
e44784b875