[llvm] [NVPTX] Legalize aext-load to zext-load to expose more DAG combines (PR #154251)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

Looks like this change accidentally removed https://github.com/llvm/llvm-project/commit/b6e19b35cd87f3167a0f04a61a12016b935ab1ea. I've added this bit back with tests to make sure this doesn't happen again. I'd appreciate both of your review on https://github.com/llvm/llvm-project/pull/155063 when you have a minute!

https://github.com/llvm/llvm-project/pull/154251