[clang][SPIR-V] Always add convergence intrinsics (#88918)

PR #80680 added bits in the codegen to lazily add convergence intrinsics
when required. This logic relied on the LoopStack. The issue is when
parsing the condition, the loopstack doesn't yet reflect the correct
values, as expected since we are not yet in the loop.

However, convergence tokens should sometimes already be available. The
solution which seemed the simplest is to greedily generate the tokens
when we generate SPIR-V.

Fixes #88144

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Signed-off-by: Nathan Gauër <brioche@google.com>