bpf: clean up visit_insn()'s instruction processing

bpf: clean up visit_insn()'s instruction processing

[ Upstream commit 653ae3a874aca6764a4c1f5a8bf1b072ade0d6f4 ]

Instead of referencing processed instruction repeatedly as insns[t]
throughout entire visit_insn() function, take a local insn pointer and
work with it in a cleaner way.

It makes enhancing this function further a bit easier as well.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230302235015.2044271-7-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Stable-dep-of: 3feb263bb516 ("bpf: handle ldimm64 properly in check_cfg()")
Signed-off-by: Sasha Levin <sashal@kernel.org>