[llvm] [SDAG] Fix CSE for ADDRSPACECAST nodes (PR #122912)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Yea, I agree it is not ideal, but I've spent some time on it and this is the best I can come up with at the moment. #121710 does rely on this change to ensure load elimination occurs for byval arguments, so once this lands we'll have much better non-debug coverage.

The 2 casts appear in the DAG if you just use -O0 for a more straightforward example:

```
define void @foo(ptr %p) {
   %a1 = addrspacecast ptr %p to ptr addrspace(5)
   %a2 = addrspacecast ptr %p to ptr addrspace(5)
   store i32 0, ptr addrspace(5) %a1
   store i32 0, ptr addrspace(5) %a2
   ret void
 }

```




https://github.com/llvm/llvm-project/pull/122912