CodeGen: Optimize arithmetics for basic multiplicative identities

This change folds:

	a * 1 => a
	a / 1 => a
	a * -1 => -a
	a / -1 => -a
	a * 2 => a + a
	a / 2^k => a * 2^-k

Note that the following folds are all invalid:

	a + 0 => a (breaks for negative zero)
	a - (-0) => a (breaks for negative zero)
	a - a => 0 (breaks for NaN)

a - 0 could be folded into a but that doesn't happen in benchmarks.

Various cases of UNM_NUM could be optimized (eg (-a) * (-b) = a * b),
but that doesn't happen in benchmarks either.

CodeGen/src/OptimizeConstProp.cpp

@@ -19,6 +19,7 @@ LUAU_FASTINTVARIABLE(LuauCodeGenReuseSlotLimit, 64)
 LUAU_FASTINTVARIABLE(LuauCodeGenReuseUdataTagLimit, 64)
 LUAU_FASTFLAGVARIABLE(DebugLuauAbortingChecks)
 LUAU_FASTFLAG(LuauVectorLibNativeDot);
+LUAU_FASTFLAGVARIABLE(LuauCodeGenArithOpt);
 
 namespace Luau
 {
@@ -1194,8 +1195,46 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
     case IrCmd::SUB_INT:
     case IrCmd::ADD_NUM:
     case IrCmd::SUB_NUM:
+        state.substituteOrRecord(inst, index);
+        break;
     case IrCmd::MUL_NUM:
+        if (FFlag::LuauCodeGenArithOpt)
+        {
+            if (std::optional<double> k = function.asDoubleOp(inst.b.kind == IrOpKind::Constant ? inst.b : state.tryGetValue(inst.b)))
+            {
+                if (*k == 1.0) // a * 1.0 = a
+                    substitute(function, inst, inst.a);
+                else if (*k == 2.0) // a * 2.0 = a + a
+                    replace(function, block, index, {IrCmd::ADD_NUM, inst.a, inst.a});
+                else if (*k == -1.0) // a * -1.0 = -a
+                    replace(function, block, index, {IrCmd::UNM_NUM, inst.a});
+                else
+                    state.substituteOrRecord(inst, index);
+            }
+            else
+                state.substituteOrRecord(inst, index);
+        }
+        else
+            state.substituteOrRecord(inst, index);
+        break;
     case IrCmd::DIV_NUM:
+        if (FFlag::LuauCodeGenArithOpt)
+        {
+            if (std::optional<double> k = function.asDoubleOp(inst.b.kind == IrOpKind::Constant ? inst.b : state.tryGetValue(inst.b)))
+            {
+                if (*k == 1.0) // a / 1.0 = a
+                    substitute(function, inst, inst.a);
+                else if (int exp = 0; frexp(*k, &exp) == 0.5 && exp >= -1000 && exp <= 1000) // a / 2^k = a * 2^-k
+                    replace(function, block, index, {IrCmd::MUL_NUM, inst.a, build.constDouble(1.0 / *k)});
+                else
+                    state.substituteOrRecord(inst, index);
+            }
+            else
+                state.substituteOrRecord(inst, index);
+        }
+        else
+            state.substituteOrRecord(inst, index);
+        break;
     case IrCmd::IDIV_NUM:
     case IrCmd::MOD_NUM:
     case IrCmd::MIN_NUM:

tests/IrLowering.test.cpp

@@ -540,7 +540,7 @@ TEST_CASE("VectorCustomAccess")
     CHECK_EQ(
         "\n" + getCodegenAssembly(R"(
 local function vec3magn(a: vector)
-    return a.Magnitude * 2
+    return a.Magnitude * 3
 end
 )"),
         R"(
@@ -560,7 +560,7 @@ bb_bytecode_1:
   %12 = ADD_NUM %9, %10
   %13 = ADD_NUM %12, %11
   %14 = SQRT_NUM %13
-  %20 = MUL_NUM %14, 2
+  %20 = MUL_NUM %14, 3
   STORE_DOUBLE R1, %20
   STORE_TAG R1, tnumber
   INTERRUPT 3u
@@ -1167,7 +1167,7 @@ local function inl(v: vector, s: number)
 end
 
 local function getsum(x)
-    return inl(x, 2) + inl(x, 5)
+    return inl(x, 3) + inl(x, 5)
 end
 )",
                    /* includeIrTypes */ true
@@ -1195,7 +1195,7 @@ bb_bytecode_1:
 bb_bytecode_0:
   CHECK_TAG R0, tvector, exit(0)
   %2 = LOAD_FLOAT R0, 4i
-  %8 = MUL_NUM %2, 2
+  %8 = MUL_NUM %2, 3
   %13 = LOAD_FLOAT R0, 4i
   %19 = MUL_NUM %13, 5
   %28 = ADD_NUM %8, %19