[HLSL][DirectX] Emit convergence control tokens when targeting DirectX (#188792)

This pr allows codegen to generate convergence control tokens. This
allows for a more accurate description of convergence behaviour to
prevent (or allow) invalid control flow graph transforms. As noted, the
use of convergence control tokens is the ideal norm and this follows
that by enabling it for `DirectX`.

This was done now under the precedent of preventing a convergent exit
condition of a loop from being illegally moved across control flow. Test
cases for this are explicitly added.

Please see the individual commits for logically similar chunks.
Unfortunately, it is tricky to stage this in smaller individual commits.

Resolves https://github.com/llvm/llvm-project/issues/180621.

https://github.com/llvm/llvm-project/pull/188537 is a pre-requisite of
this passing HLSL offload suite tests.

Assisted by: Github Copilot