LibJS/JIT: Expand `Add` fast path to double & i32 combinations

Co-authored-by: Stephan Vedder <vedder@mbits.info>

Userland/Libraries/LibJIT/X86_64/Assembler.h

@@ -878,6 +878,11 @@ struct X86_64Assembler {
             emit8(0x81);
             emit_modrm_slash(0, dst);
             emit32(src.offset_or_immediate);
+        } else if (dst.type == Operand::Type::FReg && src.type == Operand::Type::FReg) {
+            emit8(0xf2);
+            emit8(0x0f);
+            emit8(0x58);
+            emit_modrm_rm(dst, src);
         } else {
             VERIFY_NOT_REACHED();
         }

Userland/Libraries/LibJS/JIT/Compiler.cpp

@@ -625,36 +625,29 @@ void Compiler::compile_add(Bytecode::Op::Add const& op)
     load_vm_register(ARG1, op.lhs());
     load_accumulator(ARG2);
 
-    Assembler::Label end {};
-    Assembler::Label slow_case {};
-
-    branch_if_both_int32(ARG1, ARG2, [&] {
-        // GPR0 = ARG1 + ARG2 (32-bit)
-        // if (overflow) goto slow_case;
-        m_assembler.mov(
-            Assembler::Operand::Register(GPR0),
-            Assembler::Operand::Register(ARG1));
+    branch_if_both_numbers(
+        ARG1, ARG2,
+        [&](auto lhs, auto rhs, auto& slow_case) {
         m_assembler.add32(
-            Assembler::Operand::Register(GPR0),
-            Assembler::Operand::Register(ARG2),
+            Assembler::Operand::Register(lhs),
+            Assembler::Operand::Register(rhs),
             slow_case);
-
-        // accumulator = GPR0 | SHIFTED_INT32_TAG;
-        m_assembler.mov(
-            Assembler::Operand::Register(GPR1),
-            Assembler::Operand::Imm(SHIFTED_INT32_TAG));
-        m_assembler.bitwise_or(
-            Assembler::Operand::Register(GPR0),
-            Assembler::Operand::Register(GPR1));
-        store_accumulator(GPR0);
-        m_assembler.jump(end);
-    });
-
-    slow_case.link(m_assembler);
-    native_call((void*)cxx_add);
-    store_accumulator(RET);
-    check_exception();
-    end.link(m_assembler);
+        return lhs; },
+        [&](auto lhs, auto rhs) {
+            m_assembler.add(
+                Assembler::Operand::FloatRegister(lhs),
+                Assembler::Operand::FloatRegister(rhs));
+            return lhs; },
+        [&](auto lhs, auto rhs) {
+            m_assembler.mov(
+                Assembler::Operand::Register(ARG1),
+                Assembler::Operand::Register(lhs));
+            m_assembler.mov(
+                Assembler::Operand::Register(ARG2),
+                Assembler::Operand::Register(rhs));
+            native_call((void*)cxx_add);
+            return RET;
+        });
 }
 
 static Value cxx_sub(VM& vm, Value lhs, Value rhs)