[type] [refactor] Decouple quant from SNode 3/n: Extend bit pointers

[strongoier]

Related issue = #4857

The original bit pointer is like:

struct bit_pointer {
    i8 *byte_ptr;
    i32 bit_offset;
};

When creating a new bit pointer, the byte_ptr is always cast into i8 * first. Then a physical type needs to get passed alongside to allow access of its pointee.

As you may have noticed, the byte_ptr is not a pointer to a byte, but simply a byte-level pointer, which can point to any primitive type. Therefore, there is actually no need to force byte_ptr as i8 *. Then info of its physical type can be easily preserved by extending the definition of bit pointers:

// X=8/16/32/64
struct bit_pointer_X {
    iX *byte_ptr;
    i32 bit_offset;
}

After this change, logic around bit pointers can get simplified a lot.

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[k-ye]

LGTM!