[CI] Optional expensive tests.

This adds builds that runs expensive tests, including Vc on a runner with at least AVX. The build is allowed to fail to compensate for the possibility of no suitable runner being available.

added 3 commits

• 2d4e3460 - [Simd] Check for broadcast<V>(s) function.
• e90c77da - [Simd] Implement broadcast construction function.
• 18ee88af - Merge branch 'simd-broadcast-func' into 'test-vc-again'

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added 1 commit

• 24f8b9c8 - [Simd] Avoid using braces for broadcast-initialization.

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changed title from [CI] Optional tests for expensive tests. to [CI] Optional expensive tests.

changed the description

merged

mentioned in commit e995321a