bpf, arm64: Emit A64_{ADD,SUB}_I when possible in emit_{lse,ll_sc}_atomic()

Currently in emit_{lse,ll_sc}_atomic(), if there is an offset, we add it
to the base address by doing e.g.:

  if (off) {
          emit_a64_mov_i(1, tmp, off, ctx);
          emit(A64_ADD(1, tmp, tmp, dst), ctx);
  [...]

As pointed out by Xu, we can use emit_a64_add_i() (added in the previous
patch) instead, which tries to combine the above into a single A64_ADD_I
or A64_SUB_I when possible.

Suggested-by: Xu Kuohai <[email protected]>
Signed-off-by: Peilin Ye <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Xu Kuohai <[email protected]>
Link: https://lore.kernel.org/bpf/9ad3034a62361d91a99af24efa03f48c4c9e13ea.1735868489.git.yepeilin@google.com

Author: peilin-ye

arch/arm64/net/bpf_jit_comp.c

@@ -662,8 +662,7 @@ static int emit_lse_atomic(const struct bpf_insn *insn, struct jit_ctx *ctx)
 	u8 reg = dst;
 
 	if (off) {
-		emit_a64_mov_i(1, tmp, off, ctx);
-		emit(A64_ADD(1, tmp, tmp, dst), ctx);
+		emit_a64_add_i(1, tmp, reg, tmp, off, ctx);
 		reg = tmp;
 	}
 	if (arena) {
@@ -734,7 +733,7 @@ static int emit_ll_sc_atomic(const struct bpf_insn *insn, struct jit_ctx *ctx)
 	const s32 imm = insn->imm;
 	const s16 off = insn->off;
 	const bool isdw = BPF_SIZE(code) == BPF_DW;
-	u8 reg;
+	u8 reg = dst;
 	s32 jmp_offset;
 
 	if (BPF_MODE(code) == BPF_PROBE_ATOMIC) {
@@ -743,11 +742,8 @@ static int emit_ll_sc_atomic(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		return -EINVAL;
 	}
 
-	if (!off) {
-		reg = dst;
-	} else {
-		emit_a64_mov_i(1, tmp, off, ctx);
-		emit(A64_ADD(1, tmp, tmp, dst), ctx);
+	if (off) {
+		emit_a64_add_i(1, tmp, reg, tmp, off, ctx);
 		reg = tmp;
 	}