[X86] combineVectorPack - use APInt::truncSSat for PACKSS constant folding. NFC.

Unfortunately PACKUS can't use APInt::truncUSat

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -47630,16 +47630,12 @@ static SDValue combineVectorPack(SDNode *N, SelectionDAG &DAG,
           // PACKSS: Truncate signed value with signed saturation.
           // Source values less than dst minint are saturated to minint.
           // Source values greater than dst maxint are saturated to maxint.
-          if (Val.isSignedIntN(DstBitsPerElt))
-            Val = Val.trunc(DstBitsPerElt);
-          else if (Val.isNegative())
-            Val = APInt::getSignedMinValue(DstBitsPerElt);
-          else
-            Val = APInt::getSignedMaxValue(DstBitsPerElt);
+          Val = Val.truncSSat(DstBitsPerElt);
         } else {
           // PACKUS: Truncate signed value with unsigned saturation.
           // Source values less than zero are saturated to zero.
           // Source values greater than dst maxuint are saturated to maxuint.
+          // NOTE: This is different from APInt::truncUSat.
           if (Val.isIntN(DstBitsPerElt))
             Val = Val.trunc(DstBitsPerElt);
           else if (Val.isNegative())