Try to appease VS 2015 warnings from http://reviews.llvm.org/D11890

ByteSize and BitSize should not be size_t but unsigned, considering

1) They are at most 2^16 and 2^19, respectively.
2) BitSize is an argument to Type::getIntNTy which takes unsigned.

Also, use the correct utostr instead itostr and cache the string result.

Thanks to James Touton for reporting this!

llvm-svn: 245167