[clang] [X86] Emit Warnings for frontend options to enable knl/knm specific ISAs. (PR #75580)

[Freddy Ye via cfe-commits]

FreddyLeaf wrote:

> > @RKSimon @phoebewang Thanks comments! knm has AVX512_VPOPCNTDQ, I guess we also require it work without avx512vl? And we are going to keep -march/mtune/mcpu support for knl/knm, but removing some specific ISA's intrinsic and lowering supports? Did I get your point wrong?
> 
> That's correct - we keep -march/mtune/mcpu support for knl/knm but reduce its feature set to avx512f/avx512cd and we don't provide any support for xeon-phi specific features (avx512pf etc.) apart from the minimal binutils/assembly handling necessary.
> 
> If we keep support for VPOPCNTDQ on knm then yes we will need to allow non-VLX handling, but I don't know much about general availability of knm workstations (knl cpus/machines do turn up on ebay etc. quite frequently).

Thanks confirming! Modified the warning message and commit title.

https://github.com/llvm/llvm-project/pull/75580