[llvm] [X86] Make test more robust against different builds

Summary:
Rationale:
Using the --debug-only flag requires a debug build. Also, the debug output is not always consistent over different builds.
This change avoids all problems by just testing the generated assembly for AVX.

Reviewers: craig.topper, mehdi_amini, nicolasvasilache

Reviewed By: craig.topper

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78609