[X86] computeKnownBitsForPMADDWD - the final addition of the signextended multiplies is not guaranteed to be NSW

https://rust.godbolt.org/z/3f1bevnoT
https://alive2.llvm.org/ce/z/cjixBY

Noticed by @alexfh

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -37158,7 +37158,7 @@ static void computeKnownBitsForPMADDWD(SDValue LHS, SDValue RHS,
   KnownBits RHSHi = DAG.computeKnownBits(RHS, DemandedHiElts, Depth + 1);
   KnownBits Lo = KnownBits::mul(LHSLo.sext(32), RHSLo.sext(32));
   KnownBits Hi = KnownBits::mul(LHSHi.sext(32), RHSHi.sext(32));
-  Known = KnownBits::computeForAddSub(/*Add=*/true, /*NSW=*/true,
+  Known = KnownBits::computeForAddSub(/*Add=*/true, /*NSW=*/false,
                                       /*NUW=*/false, Lo, Hi);
 }