Integers with padding bits on big-endian targets cannot quite be bitcast
with a trivial memcpy, because the padding bits (which are zext or sext)
are the most-significant, so are at the *lowest* addresses. So to
bitcast to something which doesn't have padding bits, we need to offset
past the padding.

The logic I've added here definitely doesn't handle all possibilities
correctly; I think that would actually be quite complicated. However, it
handles a common case, and so prevents the Zig compiler itself from
being miscompiled on big-endian targets (hence fixing a bootstrapping
problem on big-endian).