x86_64 backend: wrong code with optionals

[xxxbxxx]

Zig Version

0.12.0-dev.2784+075e9564c

Steps to Reproduce and Observed Behavior

test {
    var v :?usize = null;
    v = 4;
    var ref : usize = 4;
    _ = &ref;
    if (v!=ref) @panic("boom");
}

fails with zig test repro.zig -target x86_64-linux-none -fno-llvm -fno-lld
but passes with zig test repro.zig -target x86_64-linux-none

Expected Behavior

the test passes.

[xxxbxxx]

I think the bug is either that
airWrapOptional() should fill the padding bits for is_not_null (it writes one byte out of 8)
or than the test should only compare the one byte and not the 7 extra padding bytes.