[llvm] [FileCheck] Fix --enable-var-scope for numvars after reassignment (PR #157158)

[via llvm-commits]

Clipi-12 wrote:

Added the comment and rephrased the PR details to that it's (hopefully) more descriptive.
(Remainder for the next review that I don't have perms, and so I cannot merge)

https://github.com/llvm/llvm-project/pull/157158