[llvm] Assembler: Replace some usage of grep in tests with FileCheck (PR #138114)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **May 1, 7:43 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/138114).


https://github.com/llvm/llvm-project/pull/138114