[clang] [clang][bytecode] Implement arithmetic, bitwise and compound assignment operator (PR #108949)

[Timm Baeder via cfe-commits]

tbaederr wrote:

Does it work to _not_ discard the result of a compound operator?

```c++


using VI __attribute__((ext_vector_type(4))) = int;

constexpr int a() {
    VI a = {0, 0, 0, 0};
    VI b = {1,1,1,1};

    VI C = (a += b);

    return 0;
}

static_assert(a() == 0);
```

https://github.com/llvm/llvm-project/pull/108949