[mlir] Rename AVX512 dialect to X86Vector

We will soon be adding non-AVX512 operations to MLIR, such as AVX's rsqrt. In https://reviews.llvm.org/D99818 several possibilities were discussed, namely to (1) add non-AVX512 ops to the AVX512 dialect, (2) add more dialects (e.g. AVX dialect for AVX rsqrt), and (3) expand the scope of the AVX512 to include these SIMD x86 ops, thereby renaming the dialect to something more accurate such as X86Vector.

Consensus was reached on option (3), which this patch implements.

Reviewed By: aartbik, ftynse, nicolasvasilache

Differential Revision: https://reviews.llvm.org/D100119