Recognize bpf_fastcall patterns around kfunc calls.
For example, suppose bpf_cast_to_kern_ctx() follows bpf_fastcall
contract (which it does), in such a case allow verifier to rewrite BPF
program below:

  r2 = 1;
  *(u64 *)(r10 - 32) = r2;
  call %[bpf_cast_to_kern_ctx];
  r2 = *(u64 *)(r10 - 32);
  r0 = r2;

By removing the spill/fill pair:

  r2 = 1;
  call %[bpf_cast_to_kern_ctx];
  r0 = r2;
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240822084112.3257995-4-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>