LibJS: Optimize x >> 0 to ToInt32 in bytecode codegen

x >> 0 is a common JS idiom equivalent to ToInt32(x). We already had
this optimization for x | 0, now do it for right shift by zero as well.

This allows the asmint handler for ToInt32 to run instead of the more
expensive RightShift handler, which wastes time loading and checking the
rhs operand and performing a shift by zero.

Libraries/LibJS/Rust/src/bytecode/codegen.rs

@@ -2077,11 +2077,29 @@ fn emit_binary_op(
             lhs: lhs_op,
             rhs: rhs_op,
         }),
-        BinaryOp::RightShift => generator.emit(Instruction::RightShift {
-            dst: dst_op,
-            lhs: lhs_op,
-            rhs: rhs_op,
-        }),
+        BinaryOp::RightShift => {
+            // OPTIMIZATION: x >> 0 == ToInt32(x) (matches C++)
+            if let Some(ConstantValue::Number(n)) = generator.get_constant(rhs) {
+                if *n == 0.0 && n.is_sign_positive() {
+                    generator.emit(Instruction::ToInt32 {
+                        dst: dst_op,
+                        value: lhs_op,
+                    });
+                } else {
+                    generator.emit(Instruction::RightShift {
+                        dst: dst_op,
+                        lhs: lhs_op,
+                        rhs: rhs_op,
+                    });
+                }
+            } else {
+                generator.emit(Instruction::RightShift {
+                    dst: dst_op,
+                    lhs: lhs_op,
+                    rhs: rhs_op,
+                });
+            }
+        }
         BinaryOp::UnsignedRightShift => generator.emit(Instruction::UnsignedRightShift {
             dst: dst_op,
             lhs: lhs_op,

Tests/LibJS/Bytecode/expected/right-shift-by-zero.txt

@@ -0,0 +1,24 @@
+$a08063d8 right-shift-by-zero.js:5:1
+  Registers: 7
+  Blocks:    1
+  Constants:
+    [0] = Undefined
+    [1] = Double(1.5)
+
+block0:
+  [   0] GetLexicalEnvironment dst:reg4
+  [   8] GetGlobal dst:reg6, `foo`
+  [  20] Call dst:reg5, callee:reg6, this_value:Undefined, foo, arguments:[Double(1.5)]
+  [  48] End value:reg5
+
+
+foo$197906fe right-shift-by-zero.js:2:5
+  Registers: 6
+  Blocks:    1
+  Constants:
+    [0] = Int32(0)
+
+block0:
+  [   0] GetLexicalEnvironment dst:reg4
+  [   8] ToInt32 dst:reg5, value:arg0
+  [  18] Return value:reg5

Tests/LibJS/Bytecode/input/right-shift-by-zero.js

@@ -0,0 +1,5 @@
+function foo(x) {
+    return x >> 0;
+}
+
+foo(1.5);