[ValueTracking] Teach canCreateUndefOrPoison that ctpop does not create undef or poison.

[craig.topper]

Summary

This select of ctpop with 0 pattern can get left behind after
loop idiom recognize converts a loop to ctpop. LLVM 10 was able
to optimize this, but LLVM 11 and later is not. The difference
seems to be that some select transforms are now limited based
on canCreateUndefOrPoison.

Teaching canCreateUndefOrPoison about ctpop restores the
LLVM 10 codegen.

[spatel]

LGTM - I'd guess that most of our intrinsics should end up in this list.

llvm/lib/Analysis/ValueTracking.cpp

case Instruction::LShr: {
     return true;
   }
   case Instruction::FPToSI:
   case Instruction::FPToUI:
     // fptosi/ui yields poison if the resulting value does not fit in the
     // destination type.
     return true;
   case Instruction::Call:
+    if (auto *II = dyn_cast<IntrinsicInst>(Op)) {
+      switch (II->getIntrinsicID()) {
+      // TODO: Add more intrinsics.
+      case Intrinsic::ctpop:
+        return false;
+      }
+    }
+    LLVM_FALLTHROUGH;
   case Instruction::CallBr:
   case Instruction::Invoke: {
     const auto *CB = cast<CallBase>(Op);
     return !CB->hasRetAttr(Attribute::NoUndef);
   }
   case Instruction::InsertElement:
   case Instruction::ExtractElement: {
     // If index exceeds the length of the vector, it returns poison

llvm/test/Transforms/InstSimplify/select.ll

 ; CHECK-NEXT:    ret i32 [[SEL]]
 ;
   %cmp = icmp ne i32 %y, 0
   %add = sub i32 %y, %x
   %sel = select i1 %cmp, i32 %add, i32 %x
   ret i32 %sel
 }
 
+define i32 @select_ctpop_zero(i32 %x) {
+; CHECK-LABEL: @select_ctpop_zero(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = call i32 @llvm.ctpop.i32(i32 [[X:%.*]])
+; CHECK-NEXT:    ret i32 [[TMP0]]
+;
+entry:
+  %0 = icmp eq i32 %x, 0
+  %1 = call i32 @llvm.ctpop.i32(i32 %x)
+  %sel = select i1 %0, i32 0, i32 %1
+  ret i32 %sel
+}
+declare i32 @llvm.ctpop.i32(i32)
+
 ; TODO: these can be optimized more
 
 define i32 @poison(i32 %x, i32 %y) {
 ; CHECK-LABEL: @poison(
 ; CHECK-NEXT:    ret i32 [[X:%.*]]
 ;
   %v = select i1 undef, i32 %x, i32 %y
   ret i32 %v