JIT: Opportunities to optimize away `ExecutionContext` saves/restores

[jakobbotsch]

We should be able to optimize away saves/restores of ExecutionContext in some cases:

1. If we can prove the ExecutionContext will not be read before we return then we can rely on the caller to restore its own ExecutionContext.
2. We can collapse "save/call/restore, save/call/restore" sequences into "save/call/restore, call/restore" sequences instead if we can prove ExecutionContext is not changed between the second "restore" and "save".

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