[llvm] [llvm-reduce] Tighten the check to count actual store NFC (PR #210933)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> It was matching `store` from source test file path and test was failing.

This test has been working fine since the beginning, and has always had store in the name. Why was it failing for you?

https://github.com/llvm/llvm-project/pull/210933