optimizer: allow multiple inlining again (#53911)
Before JuliaLang/julia#52415 we could do multiple rounds of inlining on
IR that had already been inlined, but this was no longer possible. By
removing the assertion that was introduced in JuliaLang/julia#52415,
this commit makes it possible to do multi-inlining once more. Note that
to fully solve this, though, we need to enhance `ir_inline_linetable!`
so it can add new linetables to an inner linetable that's already been
inlined. This commit notes that enhancement as something we need to do
later, leaving it with a TODO comment.
