Previously the arith folder test would emit `dense<255>` (`0xFF` zero
extended). In-memory without bytecode is `0x01`, so this change ensures
in-memory formats match.
Also changes `0xFF` to `~0x00` since compilation on machines with signed
chars was causing issues, this should ensure it is set to all ones
regardless of char interpretation:
```
[1083/5044] Building CXX object tools/mlir/lib/IR/CMakeFiles/obj.MLIRIR.dir/BuiltinDialectBytecode.cpp.o
/.../BuiltinDialectBytecode.cpp:184:35: warning: result of comparison of constant 255 with expression of type 'const char' is always false [-Wtautological-constant-out-of-range-compare]
184 | if (blob.size() == 1 && blob[0] == 0xFF) {
| ~~~~~~~ ^ ~~~~
1 warning generated.
```
Fixesllvm/llvm-project#186178
da0314842e