[NFC][LoopVectorizer] Simplify LoopVectorize/X86/gather_scatter.ll

The test used to run whole O3 pipeline. Modify it to contain LLVM IR right
before LV and limit passes to "-loop-vectorizer -simplifycfg".

For the RUN line with forced VF force interleave factor as well to simplify
CHECKs as interleaving isn't related to the purpose of the test.

I also tried to add "noalias" to pointer arguments in
@test_gather_not_profitable_pr48429 but LAI seems unable to use them.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D119786