LibJS/JIT: Add increment instruction

Instead of using `Add` with an immediate of 1 use the `Inc` instruction

Userland/Libraries/LibJIT/X86_64/Assembler.h

@@ -742,6 +742,21 @@ struct X86_64Assembler {
         }
     }
 
+    void inc32(Operand op, Optional<Label&> overflow_label)
+    {
+        if (op.is_register_or_memory()) {
+            emit_rex_for_slash(op, REX_W::No);
+            emit8(0xff);
+            emit_modrm_slash(0, op);
+        } else {
+            VERIFY_NOT_REACHED();
+        }
+
+        if (overflow_label.has_value()) {
+            jump_if(Condition::Overflow, *overflow_label);
+        }
+    }
+
     void add(Operand dst, Operand src)
     {
         if (dst.is_register_or_memory() && src.type == Operand::Type::Reg) {

Userland/Libraries/LibJS/JIT/Compiler.cpp

@@ -356,31 +356,23 @@ void Compiler::compile_increment(Bytecode::Op::Increment const&)
     Assembler::Label slow_case {};
 
     branch_if_int32(ARG1, [&] {
-        // GPR0 = ARG1 & 0xffffffff;
+        // GPR0 = ARG1
         m_assembler.mov(
             Assembler::Operand::Register(GPR0),
             Assembler::Operand::Register(ARG1));
+        // GPR0++;
+        m_assembler.inc32(
+            Assembler::Operand::Register(GPR0),
+            slow_case);
+
+        // accumulator = GPR0 | SHIFTED_INT32_TAG;
         m_assembler.mov(
             Assembler::Operand::Register(GPR1),
-            Assembler::Operand::Imm(0xffffffff));
-        m_assembler.bitwise_and(
+            Assembler::Operand::Imm(SHIFTED_INT32_TAG));
+        m_assembler.bitwise_or(
             Assembler::Operand::Register(GPR0),
             Assembler::Operand::Register(GPR1));
-
-        // if (GPR0 == 0x7fffffff) goto slow_case;
-        m_assembler.jump_if(
-            Assembler::Operand::Register(GPR0),
-            Assembler::Condition::EqualTo,
-            Assembler::Operand::Imm(0x7fffffff),
-            slow_case);
-
-        // ARG1 += 1;
-        m_assembler.add(
-            Assembler::Operand::Register(ARG1),
-            Assembler::Operand::Imm(1));
-
-        // accumulator = ARG1;
-        store_accumulator(ARG1);
+        store_accumulator(GPR0);
 
         m_assembler.jump(end);
     });