Since LowerBufferFatPointers runs before PreISelIntrinsicLowering, which
normally handles unsupported memcpy()s,, and since you can't have a
`noalias {ptr addrspace(8), i32}` becasue it crashes later passes,
manually expand memcpy()s involving buffer fat pointers to loops.
Additionally, though they're unlikely to be used, this commit adds
support for memset().
This commit doesn't implement writing direct-to-LDS loads as the
intrinsics, but leaves the option in the future.
