Rollup merge of #136758 - workingjubilee:specify-opt-level-for-tests, r=saethlin

tests: `-Copt-level=3` instead of `-O` in assembly tests

An effective blocker for redefining the meaning of `-O` is to stop reusing this somewhat ambiguous alias in our own assembly test suite. The choice between `-Copt-level=2` and `-Copt-level=3` is arbitrary for most of our tests. In most cases it makes no difference, so I set most of them to `-Copt-level=3`, as it will lead to slightly more "normalized" assembly.

Author: GuillaumeGomez