[SystemZ] Use APInt::getAllOnes()

This was using -1 without setting the signed flag.

Split off from #80309.

Author: nikic

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -7981,7 +7981,7 @@ static APInt getDemandedSrcElements(SDValue Op, const APInt &DemandedElts,
       break;
     }
     case Intrinsic::s390_vperm:
-      SrcDemE = APInt(NumElts, -1);
+      SrcDemE = APInt::getAllOnes(NumElts);
       break;
     default:
       llvm_unreachable("Unhandled intrinsic.");