[llvm] [X86][SelectionDAG] - Extend dag combiner to handle pow(1.5f), pow(x,2/3f) (PR #214202)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

What is the motivation? These forms can and should be performed in instcombine, and DAGCombine should restrict itself to patterns that arise out of codegen / legalization. I doubt anything will make pow with exact constants appear.

 I think most of these combines which can change the used library functions should probably be removed from SelectionDAG and left to IR transforms 

https://github.com/llvm/llvm-project/pull/214202