[llvm] [llvm][test] Fix filecheck annotation typos [1.5/n] (PR #94857)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Rebased, failing `llvm/test/TableGen/MixedCasedMnemonic.td`, looks like actual output changed from last time.
> 
> Need suggestions how to fix it or simply throw away fix for that test for now.

CC @s-barannikov 

https://github.com/llvm/llvm-project/pull/94857