[llvm] [HWASAN] Use sign extension in memToShadow() and untagPointer() (PR #103727)

[Florian Mayer via llvm-commits]

fmayer wrote:

Tests failed in presubmit:

```
Failed Tests (1):
  LLVM :: Instrumentation/HWAddressSanitizer/globals-access.ll
```

https://github.com/llvm/llvm-project/pull/103727