The operation `undefined & 0` ought to result in the value `0`, and likewise for
zeroing only some bits. `std/packed_int_array.zig` tests were failing because
this behavior was not implemented -- this issue was previously masked by faulty
bitcast logic which turned `undefined` values into `0xAA` on pointer loads.

Ideally, we would like to be able to track the undefined bits at comptime.
This is related to #19634.