LoongArch: BPF: Optimize sign-extention mov instructions

For 8-bit and 16-bit sign-extention mov instructions, it can use the
native instructions ext.w.b and ext.w.h directly, no need to use the
temporary t1 register, just remove the redundant operations.

Here are the test results:

  # modprobe test_bpf test_range=81,84
  # dmesg -t | tail -5
  test_bpf: #81 ALU_MOVSX | BPF_B jited:1 5 PASS
  test_bpf: #82 ALU_MOVSX | BPF_H jited:1 5 PASS
  test_bpf: #83 ALU64_MOVSX | BPF_B jited:1 5 PASS
  test_bpf: #84 ALU64_MOVSX | BPF_H jited:1 5 PASS
  test_bpf: Summary: 4 PASSED, 0 FAILED, [4/4 JIT'ed]

Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: seehearfeel

arch/loongarch/net/bpf_jit.c

@@ -527,13 +527,11 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 			emit_zext_32(ctx, dst, is32);
 			break;
 		case 8:
-			move_reg(ctx, t1, src);
-			emit_insn(ctx, extwb, dst, t1);
+			emit_insn(ctx, extwb, dst, src);
 			emit_zext_32(ctx, dst, is32);
 			break;
 		case 16:
-			move_reg(ctx, t1, src);
-			emit_insn(ctx, extwh, dst, t1);
+			emit_insn(ctx, extwh, dst, src);
 			emit_zext_32(ctx, dst, is32);
 			break;
 		case 32: