[AMDGPU] don't mark control-flow intrinsics as convergent (#90026)

This is really a workaround to allow control flow lowering in the
presence of convergence control tokens. Control-flow intrinsics in LLVM
IR are convergent because they indirectly represent the wave CFG, i.e.,
sets of threads that are "converged" or "execute in lock-step". But they
exist during a small window in the lowering process, inserted after the
structurizer and then translated to equivalent MIR pseudos. So rather
than create convergence tokens for these builtins, we simply mark them
as not convergent.

The corresponding MIR pseudos are marked as having side effects, which
is sufficient to prevent optimizations without having to mark them as
convergent.

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