[llvm] [DAGCombiner] Add some very basic folds for ADDRSPACECAST (PR #127733)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

Okay, based on both of these discussions, I've reached the conclusion that addrspacecast is not well defined enough to safely do these transformations across all targets. I've opened https://github.com/llvm/llvm-project/pull/129157 for NVPTX.

https://github.com/llvm/llvm-project/pull/127733