[TableGen][AsmMatcherEmitter] Only parse isolated tokens as registers.

Fixes PR23455, where, when TableGen generates the matcher from the
AsmString, it splits "cmp${cc}ss" into tokens, and the "ss" suffix
is recognized as the SS register.

I can't think of a situation where that's a feature, not a bug, hence:
when a token is "isolated", i.e., it is followed and preceded by
separators, it shouldn't be parsed as a register.

Differential Revision: http://reviews.llvm.org/D9844

llvm-svn: 238536