nfp: bpf: implement memory bulk copy for length within 32-bytes

For NFP, we want to re-group a sequence of load/store pairs lowered from
memcpy/memmove into single memory bulk operation which then could be
accelerated using NFP CPP bus.

This patch extends the existing load/store auxiliary information by adding
two new fields:

	struct bpf_insn *paired_st;
	s16 ldst_gather_len;

Both fields are supposed to be carried by the the load instruction at the
head of the sequence. "paired_st" is the corresponding store instruction at
the head and "ldst_gather_len" is the gathered length.

If "ldst_gather_len" is negative, then the sequence is doing memory
load/store in descending order, otherwise it is in ascending order. We need
this information to detect overlapped memory access.

This patch then optimize memory bulk copy when the copy length is within
32-bytes.

The strategy of read/write used is:

  * Read.
    Use read32 (direct_ref), always.

  * Write.
    - length <= 8-bytes
      write8 (direct_ref).
    - length <= 32-bytes and is 4-byte aligned
      write32 (direct_ref).
    - length <= 32-bytes but is not 4-byte aligned
      write8 (indirect_ref).

NOTE: the optimization should not change program semantics. The destination
register of the last load instruction should contain the same value before
and after this optimization.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>