bpf/tests: Add test of ALU shifts with operand register aliasing

This patch adds a tests of ALU32 and ALU64 LSH/RSH/ARSH operations for the
case when the two operands are the same register. Mainly intended to test
JITs that implement ALU64 shifts with 32-bit CPU instructions.

Also renamed related helper functions for consistency with the new tests.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211001130348.3670534-10-johan.almbladh@anyfinetworks.com