[llvm] Provide access to raw bits in PackedVector. NFC. (PR #98944)

[Argyrios Kyrtzidis via llvm-commits]

================
@@ -141,6 +141,8 @@ class PackedVector : public PackedVectorBase<T, BitNum, BitVectorTy,
     Bits |= RHS.Bits;
     return *this;
   }
+
+  operator BitVectorTy() const { return Bits; }
----------------
akyrtzi wrote:

Isn't this copying the underlying vector each time you want access?

How about providing reference access to it instead, maybe something like
```
const BitVectorTy &raw_bits() const { return Bits; }
```
?

https://github.com/llvm/llvm-project/pull/98944