bpf/tests: Add test of LDX_MEM with operand aliasing

This patch adds a set of tests of BPF_LDX_MEM where both operand registers
are the same register. Mainly testing 32-bit JITs that may load a 64-bit
value in two 32-bit loads, and must not overwrite the address register.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211001130348.3670534-11-johan.almbladh@anyfinetworks.com