[llvm] [SelectionDAG] Preserve cache hint metadata through DAGCombiner (PR #225954)

[Kerry McLaughlin via llvm-commits]

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kmclaughlin-arm wrote:

Does it make sense to also check that hint being added here is still the same, given that the number after `!mem.cache_hint` changes in the output?

For example:

```suggestion
; CHECK-DAG: ![[FIRST:[0-9]+]] = !{!"test.cache_hint", !"first"}
; CHECK-DAG: ![[LAST:[0-9]+]] = !{!"test.cache_hint", !"last"}
...
  ; CHECK-NEXT:   [[LDRDui:%[0-9]+]]:fpr64 = LDRDui [[COPY]], 0 :: (load (s64) from %ir.p, align 1, !mem.cache_hint ![[FIRST]])
```

https://github.com/llvm/llvm-project/pull/225954