[llvm] [IPO] Added attributor for identifying invariant loads (PR #141800)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> If we want to get rid of the constant address space idiom, that's fine by me - we'll just want to get the frontends to tag their loads from constants as invariant

The main problem is invariant load doesn't do anything on memcpy or other memory intrinsics 

https://github.com/llvm/llvm-project/pull/141800