[Mangler] Fix a bug in the bit width calculation of numbers representing strings.

The bug is that zext expects the bitwidth to be different, which is incorrect
since the bitwidth does not change most of the times.

Author: nadavrot

lib/ABI/Compression.cpp

@@ -151,7 +151,7 @@ static char DecodeFixedWidth(APInt &num) {
 
   APInt Quotient(BW, 0), Remainder(BW, 0);
   APInt::udivrem(num, C, Quotient, Remainder);
-  num = Quotient.zext(BW);
+  num = Quotient.zextOrSelf(BW);
   return Huffman::Charset[Remainder.getZExtValue()];
 }