[APInt] Make behavior of ashr by BitWidth consistent between single and multi word.

Previously single word would always return 0 regardless of the original sign. Multi word would return all 0s or all 1s based on the original sign. Now single word takes into account the sign as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301159 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/APInt.cpp

@@ -1041,7 +1041,9 @@ APInt APInt::ashr(unsigned shiftAmt) const {
   // Handle single word shifts with built-in ashr
   if (isSingleWord()) {
     if (shiftAmt == BitWidth)
-      return APInt(BitWidth, 0); // undefined
+      // Undefined
+      return APInt(BitWidth,
+                   SignExtend64(VAL, BitWidth) >> (APINT_BITS_PER_WORD - 1));
     return APInt(BitWidth, SignExtend64(VAL, BitWidth) >> shiftAmt);
   }