[clang] [clang] Emit @llvm.assume before streaming_compatible functions when the streaming mode is known (PR #121917)

[Sander de Smalen via cfe-commits]

sdesmalen-arm wrote:

@efriedma-quic a motivating example is https://godbolt.org/z/aW9zTrdf9

With @NickGuy-Arm's patch, this would compile to:
```
_Z1xv:
        rdvl    x0, #1
        ret

_Z1yv:
        rdsvl   x0, #1
        ret
```

https://github.com/llvm/llvm-project/pull/121917