[LowerMemIntrinsics] Explicitly use i8 type in memmove lowering

By convention, memcpy/memmove intrinsics are always used with i8
pointers (though this is not enforced), so in practice this code
was always using an i8 type. Make that explicit.

Of course, i8 is not a very profitable choice, and this code could
be more performant by picking an appropriate larger type. But that
would require additional test coverage and correctness review, and
certainly shouldn't be a decision based on the pointer element type.