[llvm] Update documentation on x86 constraint codes (PR #68830)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Hi! Sorry for being late here.

Curiously, https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html doesn't make such distinctions about `x` constraint wrt AVX512.

https://github.com/llvm/llvm-project/pull/68830