bpf: Add more check directives in peephole testcase

Improve the test accuracy by adding more check directives.

Shifts are expected to be eliminated for zero extension but not for signed
extension.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 327364