[llvm] [BPF] Implement `canLowerReturn` for big returns (PR #174478)

[via llvm-commits]

yonghong-song wrote:

For this one:
> The int128 return will be used in internal functions only. CanLowerReturn will save the 16 bytes of i128
> numbers in a pointer instead of using the r0 register, which only holds 8 bytes.

Could you illustrate how this could be used by internal functions? It would be great if you have more details on this in description above.

https://github.com/llvm/llvm-project/pull/174478