[ValueTracking] If `OrZero` is set, look through `trunc` in `isKnownToBePowerOfTwo`

[goldstein.w.n]

Summary

Just move coverage.

Proof: https://alive2.llvm.org/ce/z/H37tVX

[nikic]

LGTM

[goldstein.w.n]

Rebase

[goldstein.w.n]

Rebase

llvm/lib/Analysis/ValueTracking.cpp

bool isKnownToBeAPowerOfTwo(const Value *V, bool OrZero, unsigned Depth,
 
   // The remaining tests are all recursive, so bail out if we hit the limit.
   if (Depth++ == MaxAnalysisRecursionDepth)
     return false;
 
   switch (I->getOpcode()) {
   case Instruction::ZExt:
     return isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
+  case Instruction::Trunc:
+    return OrZero && isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
   case Instruction::Shl:
     if (OrZero || Q.IIQ.hasNoUnsignedWrap(I) || Q.IIQ.hasNoSignedWrap(I))
       return isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
     return false;
   case Instruction::LShr:
     if (OrZero || Q.IIQ.isExact(cast<BinaryOperator>(I)))
       return isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
     return false;

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

 
 }
 
 define i8 @trunc_is_pow2_or_zero(i16 %x, i8 %y) {
 ; CHECK-LABEL: define i8 @trunc_is_pow2_or_zero
 ; CHECK-SAME: (i16 [[X:%.*]], i8 [[Y:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = trunc i16 [[XP2]] to i8
-; CHECK-NEXT:    [[R:%.*]] = urem i8 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i8 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %xp2 = shl i16 4, %x
   %xx = trunc i16 %xp2 to i8
 
   %r = urem i8 %y, %xx
   ret i8 %r
 }