When initializing a packed struct, we must ensure the result local
is zero'd. Previously we would do this by ensuring a new local is
allocated. Although a local is always zero by default, it meant that
if such an initialization was being done inside a loop, it would re-
use that very same local that could potentially still hold a different
value. Because this value is `or`'d with the value, it would result
in a miscompilation. By manually setting this result to 0, we guarantee
the correct behavior.