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

[via cfe-commits]

yronglin 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);
> ```

Good catch! I've fixed this issue.

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