[llvm] [NFC][VPlan] Add extra tests for VPlan-based stride speculation (PR #218795)

[Andrei Elovikov via llvm-commits]

eas wrote:

> Some of the test files have grown quite large, it might be worth considering splitting them up across some common themes

I'll drop half the CHECK/RUN lines in the biggest file after the PR will get merged as they're only duplicated for the review purpose. I also think that splitting test files (especially artificially) reduces maintainability/readability, not improves it. When studying some (new to me) feature I'd rather have all tests in a single place, then scattered across multiple files.

https://github.com/llvm/llvm-project/pull/218795