sparse: Also check bit_offset when checking implicit casts

sparse: Also check bit_offset when checking implicit casts

The comparison in same_cast_type only checked the bit_size of the new and the
old symbol. The bit_offset was only compared with itself and thus would always
be true. Instead Linus most likely wanted to compare the bit_offset of the new
and the old symbol. This regression was introduced in
47f53396a1d62719c44941f84370ead80181728e ("If two implied casts end up undoing
each other, just remove them.").

Reported-by: James Westby <jw+debian@jameswestby.net>
Signed-off-by: Franz Schrober <franzschrober@yahoo.de>
Signed-off-by: Christopher Li <sparse@chrisli.org>