[APInt] Use APINT_BITS_PER_WORD instead of recomputing it. NFC

Author: topperc

llvm/lib/Support/APInt.cpp

@@ -502,10 +502,10 @@ uint64_t APInt::extractBitsAsZExtValue(unsigned numBits,
   if (loWord == hiWord)
     return (U.pVal[loWord] >> loBit) & maskBits;
 
-  static_assert(8 * sizeof(WordType) <= 64, "This code assumes only two words affected");
-  unsigned wordBits = 8 * sizeof(WordType);
+  static_assert(APINT_BITS_PER_WORD <= 64,
+                "This code assumes only two words affected");
   uint64_t retBits = U.pVal[loWord] >> loBit;
-  retBits |= U.pVal[hiWord] << (wordBits - loBit);
+  retBits |= U.pVal[hiWord] << (APINT_BITS_PER_WORD - loBit);
   retBits &= maskBits;
   return retBits;
 }