[InstCombine] Drop poison generating flags on Or in simplifyAssocCastAssoc.

Fixes #74739.

Author: topperc

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -356,6 +356,7 @@ static bool simplifyAssocCastAssoc(BinaryOperator *BinOp1,
 
   IC.replaceOperand(*Cast, 0, BinOp2->getOperand(0));
   IC.replaceOperand(*BinOp1, 1, FoldedC);
+  BinOp1->dropPoisonGeneratingFlags();
   Cast->dropPoisonGeneratingFlags();
   return true;
 }

llvm/test/Transforms/InstCombine/or.ll

@@ -1609,11 +1609,11 @@ define i8 @drop_disjoint(i8 %x) {
   ret i8 %b
 }
 
-; FIXME: We should drop disjoint here.
+; Make sure we drop disjoint when combining the Ors.
 define i32 @assoc_cast_assoc_disjoint(i16 %x) {
 ; CHECK-LABEL: @assoc_cast_assoc_disjoint(
 ; CHECK-NEXT:    [[B:%.*]] = zext i16 [[X:%.*]] to i32
-; CHECK-NEXT:    [[C:%.*]] = or disjoint i32 [[B]], 65537
+; CHECK-NEXT:    [[C:%.*]] = or i32 [[B]], 65537
 ; CHECK-NEXT:    ret i32 [[C]]
 ;
   %a = or i16 %x, 1