[SafeStack] Fix crashing with scalable TypeSizes (#180547)
On e.g. aarch64 the TypeSize of scalar types can have a size that is not
known at compile time.
Currently when safestack occurs those it simply crashes as described in
https://github.com/llvm/llvm-project/issues/175868.
Since we cannot verify the size at compile time we simply consider the
access to be unsafe (in regards to safestack).
Reproducer:
```
#include <arm_sve.h>
int main() {
svint32_t vec = svindex_s32(0, 1);
svint32_t res = svadd_s32_z(svptrue_b32(), vec, vec);
int32_t buffer[1024];
svst1_s32(svptrue_b32(), buffer, res);
return 0;
}
```
