[llvm] [SPIR-V] Fix signed-shift UB when packing UTF-8 string operands (PR #207769)

[Juan Manuel Martinez Caamaño via llvm-commits]

================
@@ -161,7 +161,7 @@ static uint32_t convertCharsToWord(const StringRef &Str, unsigned i) {
     if (StrIndex < Str.size()) { // If it's within the string, get a real char.
       CharToAdd = Str[StrIndex];
     }
-    Word |= (CharToAdd << (WordIndex * 8));
+    Word |= (static_cast<uint32_t>(CharToAdd) << (WordIndex * 8));
   }
   return Word;
 }
----------------
jmmartinez wrote:

Nice catch!

But isn't this just a complicated `memcpy`?

```cpp
static uint32_t convertCharsToWord(StringRef Str, unsigned i) {
  uint32_t Word = 0;
  unsigned Count = std::min(sizeof(Word), Str.size() - i);
  std::memcpy(&Word, &Str[i], Count);
  return Word;
}
```

https://github.com/llvm/llvm-project/pull/207769