[ValueTracking] Add support for `mul` in `isKnownToBeAPowerOfTwo`

[goldstein.w.n]

Summary

pow2 * pow2 is a power of 2 or zero.

Proof: https://alive2.llvm.org/ce/z/FNiiXd

[nikic]

LGTM

[goldstein.w.n]

Rebase

[goldstein.w.n]

Rebase

llvm/lib/Analysis/ValueTracking.cpp

bool isKnownToBeAPowerOfTwo(const Value *V, bool OrZero, unsigned Depth,
   case Instruction::LShr:
     if (OrZero || Q.IIQ.isExact(cast<BinaryOperator>(I)))
       return isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
     return false;
   case Instruction::UDiv:
     if (Q.IIQ.isExact(cast<BinaryOperator>(I)))
       return isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
     return false;
+  case Instruction::Mul:
+    return OrZero &&
+           isKnownToBeAPowerOfTwo(I->getOperand(1), OrZero, Depth, Q) &&
+           isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
   case Instruction::And:
     if (OrZero) {
       // A power of two and'd with anything is a power of two or zero.
       if (isKnownToBeAPowerOfTwo(I->getOperand(0), /*OrZero*/ true, Depth, Q) ||
           isKnownToBeAPowerOfTwo(I->getOperand(1), /*OrZero*/ true, Depth, Q))
         return true;
       // X & (-X) is always a power of two or zero.
       if (match(I->getOperand(0), m_Neg(m_Specific(I->getOperand(1)))) ||

llvm/test/Analysis/ValueTracking/known-power-of-two.ll

 }
 
 define i16 @mul_is_pow2_or_zero(i16 %x, i16 %y, i16 %z) {
 ; CHECK-LABEL: define i16 @mul_is_pow2_or_zero
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]], i16 [[Z:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[ZP2:%.*]] = shl i16 2, [[Z]]
 ; CHECK-NEXT:    [[XX:%.*]] = mul i16 [[XP2]], [[ZP2]]
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
   %zp2 = shl i16 2, %z
   %xx = mul i16 %xp2, %zp2
 
   %r = urem i16 %y, %xx
   ret i16 %r