[llvm] [GISel] Explicitly disable BF16 tablegen patterns. (PR #124113)

[Sjoerd Meijer via llvm-commits]

https://github.com/sjoerdmeijer approved this pull request.

This fixes a codegen/runtime error that we are seeing, as reported in https://github.com/llvm/llvm-project/pull/120363#issuecomment-2609616562

I am not a GISel guy, but I have applied and verified that this patch solves the problems that we are seeing, and the change looks very reasonable, so am confident to LGTM this. 

Thanks for the quick fix!

https://github.com/llvm/llvm-project/pull/124113