[clang] [clang][x86] Fix `_mm512_popcnt_epi64` constexpr test (PR #152478)

[Pedro Lobo via cfe-commits]

pedroclobo wrote:

> nice catch - any idea why the test was passing?

I don't really know how to run/verify these `constexpr` tests. At least with `ninja check-all`, they don’t seem to run in a way that clearly reports the `static_assert` failures.

I've been relying on my editor's LSP. It flagged the failing `static_assert`, which is how I caught the issue.

https://github.com/llvm/llvm-project/pull/152478