"llvm-as is an LLVM IR -> LLVM bitcode assembler not a system
assembler"[1], and therefore should not be linked as "as".
The "as" symlink was removed in 46e5ea5af6 ("llvm*: remove symlinks
to llvm-diff, llvm-as and associated LLVM IR utilities."), but that
was partially reverted by b331c72f03 ("llvm: setup some symlinks for
compatibility with binutils"), which restored a bunch of symlinks that
were incorrectly removed, but also incorrectly restored "as". This
was pointed out[2] at the time but apparently never fixed.
[1]: https://github.com/NixOS/nixpkgs/issues/93523#issue-661663683
[2]: b331c72f03 (commitcomment-40981705)
a5e1363258