[llvm] [GlobalISel] Fall back for bf16 conversions. (PR #71470)

[via llvm-commits]

erickq wrote:

Now an error is reported for a very simple bf16 data addition. Is this modification too violent?
```
#include <arm_neon.h>
bfloat16_t test(bfloat16_t gpara1, bfloat16_t gpara2)
{
    return gpara1+gpara2;
}
```

https://github.com/llvm/llvm-project/pull/71470