Fix C++0x narrowing errors when char is unsigned.

Fix C++0x narrowing errors when char is unsigned.

In the case of EDInstInfo, this would actually cause a bug when -1 became 255
and was then compared >=0 in llvm-mc/Disassembler.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138825 91177308-0d34-0410-b5e6-96231b3b80d8