[llvm] [TLI] Make VecDesc statically initializable (PR #211307)

[Valery Chernov via llvm-commits]

vvchernov wrote:

> > Can you mark the tables `constinit` to prevent regressions? This might require changing tablegen.
> 
> Is this feasible? I think it's a trivial tablegen change. If it doesn't work, it doesn't work.

Hello @rnk! `constinit` isn't supported by C++17 which is default for LLVM now. So I changed `const` to `constexpr` for VecDesc tables in `TargetLibraryInfo.cpp` and added comment about change it into `constinit` when LLVM requires C++20. Correct me if I wrongly caught your idea.
Speaking of tablegen: the VecFuncs_* tables are built from VecFuncs.def not TableGen, so no changes are needed.

https://github.com/llvm/llvm-project/pull/211307