[llvm] [Analysis] Avoid repeated hash lookups (NFC) (PR #128394)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@nikic Thanks for the review!  I don't think my original PR was correct.  Without my PR, we update the map in every iteration:

```
ObjToLastAccess[UnderlyingObj] = Access;
```

In the latest revision, I now update the map even when the lookup succeeds.

If you could review this PR again, that would be greatly appreciated.  Text "LGTM" would be fine.  Thanks!

https://github.com/llvm/llvm-project/pull/128394