riscv, bpf: Fix missing exception handling and redundant zext for LDX_B/H/W

riscv, bpf: Fix missing exception handling and redundant zext for LDX_B/H/W

For LDX_B/H/W, when zext has been inserted by verifier, it'll return 1,
and no exception handling will continue. Also, when the offset is 12-bit
value, the redundant zext inserted by the verifier is not removed. Fix
both scenarios by moving down the removal of redundant zext.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20230824095001.3408573-2-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>