[llvm] GlobalISel needs fdiv 1 / sqrt(x) to rsq combine (PR #78673)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I think this is close. I forgot that I remove the codegen support for rsq in the DAG over the summer. I think this patch only needs to handle the f16 part, and not introduce f32/f64 rsq. We do that in CodeGenPrepare 

https://github.com/llvm/llvm-project/pull/78673