This PR introduces `amdgpu-lower-exec-sync` pass which specifically
lowers named-barrier LDS globals introduced by #114550 .

Changes include:

- Moving the logic of lowering named-barrier LDS globals from
`amdgpu-lower-module-lds` pass to this new pass.

- This PR adds the pass to pipeline, remove the existing lowering logic for
named-barrier LDS in `amdgpu-lower-module-lds`

See #161827 for discussion on this topic.

We just replaced SmallSet<T *, N> with SmallPtrSet<T *, N>, bypassing
the redirection found in SmallSet.h.  With that, we no longer need to
include SmallSet.h in many files.

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 140 instances that rely on this "redirection", with the
vast majority of them under llvm/. Since relying on the redirection
doesn't improve readability, this patch replaces SmallSet with
SmallPtrSet for pointer element types.

Move canGuaranteeTCO and mayTailCallThisCC into AMDGPUBaseInfo instead
of keeping two copies for DAG/Global ISel.

Also remove isKernelCC, which doesn't agree with isKernel and doesn't
seem very useful.

While at it, also move all the CC-related helpers into AMDGPUBaseInfo.h and
mark them constexpr.

I think it was an accident that this wasn't included.