UndefinedBehaviorSanitizer: false-positive on arm64

[lewis6991]

I've found when using -fsanitize=undefined on a Macbook M1 Pro machine with the following code, the sanitizer thinks we are indexing the array with a negative value. I do not get this result when running on x86_64

int main(void)
{
  int a[256] = { 0 };
  int b = 0x80;
  int c = a[b & 0xff];
}

clang -g -arch arm64 -fsanitize=undefined test.c -o test

test.c:5:11: runtime error: index -128 out of bounds for type 'int[256]'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test.c:5:11 in