Support PreIselIntrinsicLowering of memory intrinsics

[arichardson]

Since llvm/llvm-project@3c848194f28d building with no-builtin will expand memory intrinsics in IR instead of going to the default SelectionDAG IR. This allows expanding non-constant size calls but currently does not support CHERI since it copies integers.

I disabled this in the LLVM 17 merge, but we might want to support it so we can generate -fno-builtin code that does not have references to memcpy/memmove.
Unlike GPU usecases those libcalls don't matter too much, so filing this as an enhancement.