[AMD] Use DenseMap in CanonicalizePointers FatPtrAttrs (#5889)

This PR fixes an assert thrown in `DenseMap::copyFrom`
that looks like this:

```
... Assertion `getNumBuckets() == other.getNumBuckets()' failed.
```

The call to `copyFrom` is via `SmallDenseMap`s copy ctor which fires in
our code in the default ctor of `CanonicalizePointers::FatPointers::FatPtrAttrs`
since `FatPtrAttrs` wraps a `llvm::SmallDenseMap<StringRef, Attribute>
attributes`.

I have locally verified the fix resolves the issue .

Author: makslevental

third_party/amd/lib/TritonAMDGPUTransforms/CanonicalizePointers.cpp

@@ -296,7 +296,7 @@ struct FatPointers {
       return !(lhs == rhs);
     }
 
-    llvm::SmallDenseMap<StringRef, Attribute> attributes;
+    llvm::DenseMap<StringRef, Attribute> attributes;
     bool canNarrow = false;
   };