[llvm] [IR] Add MemsetPatternInst to the MemIntrinsic hierarchy (PR #153301)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> You okay if I defer this API design to a patch which needs to reason about the length in bytes? Or would you rather I speculative add it now?

The latter one.

https://github.com/llvm/llvm-project/pull/153301