[APInt] Slightly simplify APInt::ashrSlowCase. NFC (#111220)

Use an arithmetic shift for the last word copy when BitShift!=0. This
avoids an explicit sign extend after the shift.

Author: topperc

llvm/lib/Support/APInt.cpp

@@ -1055,11 +1055,10 @@ void APInt::ashrSlowCase(unsigned ShiftAmt) {
         U.pVal[i] = (U.pVal[i + WordShift] >> BitShift) |
                     (U.pVal[i + WordShift + 1] << (APINT_BITS_PER_WORD - BitShift));
 
-      // Handle the last word which has no high bits to copy.
-      U.pVal[WordsToMove - 1] = U.pVal[WordShift + WordsToMove - 1] >> BitShift;
-      // Sign extend one more time.
+      // Handle the last word which has no high bits to copy. Use an arithmetic
+      // shift to preserve the sign bit.
       U.pVal[WordsToMove - 1] =
-          SignExtend64(U.pVal[WordsToMove - 1], APINT_BITS_PER_WORD - BitShift);
+          (int64_t)U.pVal[WordShift + WordsToMove - 1] >> BitShift;
     }
   }