nfp: bpf: correct the behavior for shifts by zero

nfp: bpf: correct the behavior for shifts by zero

Shifts by zero do nothing, and should be treated as nops.

Even though compiler is not supposed to generate such instructions and
manual written assembly is unlikely to have them, but they are legal
instructions and have defined behavior.

This patch correct existing shifts code-gen to make sure they do nothing
when shift amount is zero except when the instruction is ALU32 for which
high bits need to be cleared.

For shift amount bigger than type size, already, NFP JIT back-end errors
out for immediate shift and only low 5 bits will be taken into account for
indirect shift which is the same as x86.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>