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

Try to appease VS 2015 warnings from 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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245167 91177308-0d34-0410-b5e6-96231b3b80d8