[llvm] LoopVectorize/test: fix opt invocations with -march (NFC) (PR #74462)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

> opt erroneously accepts the -march command-line argument, although this argument has no effect. 

I don't think this is quite right. I believe `-march` will still be used when constructing the TargetMachine. But it is only meaningful in conjunction with a triple, and in the cases changed here it is entirely redundant.

https://github.com/llvm/llvm-project/pull/74462