[llvm] [LLVM][IR] Teach extractelement folds about constant ConstantInt/FP. (PR #116793)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

NOTE: This doesn't fix up all folds but I'm trying to make use of existing tests where possible before circling back to add new ones required for future fixes. 

https://github.com/llvm/llvm-project/pull/116793