JIT: Allow forward subbing no-return calls

[jakobbotsch]

The comment refers to morph getting tripped up, but that should not be the case anymore after #105942.

[Copilot]

Pull Request Overview

This PR updates the forward substitution logic to also block GT_ASYNC_CONTINUATION nodes and removes the special-case guard against no-return calls, allowing them to be substituted.

• Added GT_ASYNC_CONTINUATION to the substitution guard.
• Removed the no-return call check so forward subbing will include calls marked no-return.
• Updated the guard comment to reflect the new set of blocked node types.

Comments suppressed due to low confidence (2)

src/coreclr/jit/forwardsub.cpp:505

• Update the JITDUMP message to include async continuations (e.g., 'tree to sub is catch arg, lcl heap, or async continuation') so the debug output matches the new guard comment.

JITDUMP(" tree to sub is catch arg, or lcl heap\n");

src/coreclr/jit/forwardsub.cpp:508

• Add a unit or integration test case to verify that forward substitution now correctly handles no-return calls, since the previous guard was removed.

// Do not substitute async calls; if the target node has a temp BYREF node,

[AndyAyersMS]

We should be more aggressive in tree pruning when we see no return calls, but we should do that sometime earlier.

[jakobbotsch]

/ba-g Timeout is #115955