This issue starts in the selection DAG and causes the backend to emit
the following for a trivial tail call:
```
ldr w8, [sp]
str w8, [sp]
b func
```
I'm not too sure that checking for immutability of a specific stack
object is a good enough of a gurantee, because as soon a tail-call is
done lowering,`setHasTailCall()` is called and in that case perhaps a
pass is allowed to change the value of the object in-memory?
This can be extended to the ARM backend as well.
Removed the `tailcall` keyword from a few other test assets, I'm
assuming their original intent was left intact.
4d4b7cc69e