[BUG] Store on bitcasted unsafe pointer not working as expected

[martinvuyk]

Bug description

What I'm trying to achieve is to force it to interpret the buffer of uint8 as uint64 to only use 1 xor operation for 8+ values at once

This should xor each value with 0x30 and get the num value out of the string but the pointer doesn't edit the values

fn atol_utf8[min_str_len: Int](owned str_ref: StringRef) -> UInt64:
    """Parses the given utf-8 string assuming the string is clean and contains
    only number characters: [0, 9].

    For example, `atol_utf8[2]("19")` returns `19`. If the given string cannot be parsed
    as an integer value, garbage is returned e.g. `atol_utf8[2](" 19")` returns `1619`
    `atol_utf8[2]("-19")` returns `2919`.

    Parameters:
        min_str_len: The minimum length that the string must have.

    Args:
        str_ref: A utf-8 string to be parsed as an integer.

    Returns:
        An integer value that represents the string.
    """
    var ptr = str_ref.unsafe_ptr().bitcast[DType.uint64]()

    var str_len = len(str_ref)
    alias factor = 8
    alias amnt_full = min_str_len // factor

    for i in range(amnt_full):
        # this should edit the buffer but it doesn't
        ptr[i] ^= 0x3030303030303030

    var ptr_8 = ptr.bitcast[DType.uint8]()
    var int_res = UInt64(0)

    for i in range(str_len):
        # manually doing xor for each works
        # print(value ^ 0x30)
        var value = ptr_8[i].cast[DType.uint64]()
        if i >= amnt_full * factor:
            value ^= 0x30
        int_res = int_res * 10 + value

    return int_res


fn main():
    var result = atol_utf8[9]("123456789")
    print(result) # should print `123456789` but prints `5456790069` which xored to 0x30 is `123456789`

Steps to reproduce

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System information

- What OS did you do install Mojo on ?
Ubuntu 22
- Provide version information for Mojo by pasting the output of `mojo -v`
mojo 2024.5.1619
- Provide Modular CLI version by pasting the output of `modular -v`