[llvm] TableGen: Replace assertion with error for unexpected pattern inputs (PR #159687)

[Sergei Barannikov via llvm-commits]

https://github.com/s-barannikov approved this pull request.

I was thinking of supporting this syntax on top-level patterns, but trying to implement it revealed several issues that are not trivial to fix.

https://github.com/llvm/llvm-project/pull/159687