[asan] Fix shadow load alignment for unaligned 128-bit load/store

When a 128-bit load/store is aligned by 8, we incorrectly emit `load i16, ptr ..., align 2`
while the shadow memory address may not be aligned by 2.

This manifests as possibly-misaligned shadow memory load with `-mstrict-align`,
e.g. `clang --target=aarch64-linux -O2 -mstrict-align -fsanitize=address`
```
__attribute__((noinline)) void foo(unsigned long *ptr) {
  ptr[0] = 3;
  ptr[1] = 3;
}
// ldrh    w8, [x9, x8]  // the shadow memory load may not be aligned by 2
```

Infer the shadow memory alignment from the load/store alignment to set the
correct alignment. The generated code now uses two ldrb and one orr.

Fix https://github.com/llvm/llvm-project/issues/63258

Differential Revision: https://reviews.llvm.org/D152663