bpf: add selftests for lsh, rsh, arsh with reg operand

Current tests cover only shifts with an immediate as the source
operand/shift counts; add a new test case to cover register operand.

Signed-off-by: Jie Meng <jmeng@fb.com>
Link: https://lore.kernel.org/r/20221007202348.1118830-4-jmeng@fb.com


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