bpf, x64: save one byte per shl/shr/sar when imm is 1

When we shift by one, we can use a different encoding where imm
is not explicitly needed, which saves 1 byte per such op.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>