Change SMRange to be half-open (exclusive end) instead of closed (inclusive)

This is necessary not only for representing empty ranges, but for handling
multibyte characters in the input. (If the end pointer in a range refers to
a multibyte character, should it point to the beginning or the end of the
character in a char array?) Some of the code in the asm parsers was already
assuming this anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171765 91177308-0d34-0410-b5e6-96231b3b80d8