MSVC's implementation of isalnum will assert on characters > 255, so we need to use...

MSVC's implementation of isalnum will assert on characters > 255, so we need to use an unsigned char to ensure the integer promotion happens properly.  This fixes an assert in debug builds with CodeGen\X86\utf8.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160286 91177308-0d34-0410-b5e6-96231b3b80d8