[X86][AVX512FP16] Relax limitation to AVX512FP16 intrinsics. NFCI

Since we have enabled the support for `_Float16` on SSE2, we can relax
the limitation for AVX512FP16 now. This helps for user to use AVX512FP16
mixed with unsupported versions, e.g., multiversioning.

Also fix lit fails due to missing const modifier. Found during this change.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D132342