selftests/bpf: Add reg_bounds tests for ldsx and subreg compare

Add a few reg_bounds selftests to test 32/16/8-bit ldsx and subreg comparison.
Without the previous patch, all added tests will fail.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240723162940.2732171-1-yonghong.song@linux.dev


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