[Mangler] Fix a bug in EncodeStringAsNumber.

When we encode a string into an APInt the size of the APInt needs to be large
enough for the string (we got this part right), but also large enough for
arithmetic with the value of character length.

Author: nadavrot

lib/ABI/Compression.cpp

@@ -144,12 +144,14 @@ std::string swift::Compress::EncodeCBCString(StringRef In) {
 }
 
 static char DecodeFixedWidth(APInt &num) {
-  unsigned BW = std::max(num.getBitWidth(), 32u);
+  unsigned BW = num.getBitWidth();
+  assert(BW > 16 &&
+         "The APInt needs to be large enough for arithmetic on CharsetLength");
   APInt C = APInt(BW, Huffman::CharsetLength);
 
   APInt Quotient(BW, 0), Remainder(BW, 0);
   APInt::udivrem(num, C, Quotient, Remainder);
-  num = Quotient;
+  num = Quotient.zext(BW);
   return Huffman::Charset[Remainder.getZExtValue()];
 }
 
@@ -169,7 +171,7 @@ static void EncodeFixedWidth(APInt &num, char ch) {
 
 APInt
 swift::Compress::EncodeStringAsNumber(StringRef In, EncodingKind Kind) {
-  unsigned BW = std::max(1u, (unsigned) In.size() *
+  unsigned BW = std::max(32u, (unsigned) In.size() *
                          Huffman::LongestEncodingLength);
     APInt num = APInt(BW, 0);