[llvm] [VectorUtils] Simplify the code by new function InterleaveGroup::isFull. nfc (PR #151112)

[Mel Chen via llvm-commits]

================
@@ -633,6 +633,9 @@ template <typename InstTy> class InterleaveGroup {
     return true;
   }
 
+  /// Return true if the group has no gaps.
+  bool isFull() const { return getNumMembers() == getFactor(); }
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Mel-Chen wrote:

hasNoGaps sounds more specific, but the comments mentioned 'full group'. Maybe we can still use isFull, but improve the comment instead. What do you think?
```
/// Return true if group is full, i.e. it has no gaps.
```

https://github.com/llvm/llvm-project/pull/151112