GlobalISel: Lower non-byte loads and stores

Previously we didn't preserve the memory type and had to blindly
interpret a number of bytes. Now that non-byte memory accesses are
representable, we can handle these correctly.

Ported from DAG version (minus some weird special case i1 legality
checking which I don't fully understand, and we don't have a way to
query for)

For now, this is NFC and the test changes are placeholders. Since the
legality queries are still relying on byte-flattened memory sizes, the
legalizer can't actually see these non-byte accesses. This keeps this
change self contained without merging it with the larger patch to
switch to LLT memory queries.