bpf: implement insn_is_cast_user() helper for JITs

Implement a helper function to check if an instruction is
addr_space_cast from as(0) to as(1). Use this helper in the x86 JIT.

Other JITs can use this helper when they add support for this instruction.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Link: https://lore.kernel.org/r/20240324183226.29674-1-puranjay12@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>