Add OpenMP SIMD pragmas to some loops

[imciner2]

OpenMP 4.5 introduced the #pragma omp simd annotation to tell the compiler a loop can be vectorized using SIMD instructions. It doesn't hurt to have it, since the pragmas are just ignored when disabled, and we can compile with only the OpenMP SIMD support and not get the whole OpenMP runtime, so there is no additional dependency.

I don't know if this makes a large difference in QDLDL itself when doing a single solve, but I think this is something that will help OSQP with its multiple solves (since the SIMD annotations are all on the solve phase).

[coveralls]

Pull Request Test Coverage Report for Build 9024192663

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8940488794:	0.0%
Covered Lines:	88
Relevant Lines:	88

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💛 - Coveralls