[TableGen] Treat reg as isolated in reg$foo (but not in ${foo}reg)

D9844 fixed a problem where the ss suffix in the AsmString "cmp${cc}ss"
was recognised as the X86 SS register, by only recognising a token as a
register name if it is "isolated", i.e. surrounded by separator
characters.

In the AMDGPU backend there are many operands like $clamp which expand
to an optional string " clamp" including the preceding space, so we want
to have AsmStrings including sequences like "vcc$clamp" where vcc is a
register name.

This patch relaxes the rules for an isolated token, to say that it's OK
if the token is immediately followed by a '$'.

Differential Revision: https://reviews.llvm.org/D90315

llvm/utils/TableGen/AsmMatcherEmitter.cpp

@@ -1018,7 +1018,7 @@ void MatchableInfo::tokenizeAsmString(const AsmMatcherInfo &Info,
 
     case '$': {
       if (InTok) {
-        addAsmOperand(String.slice(Prev, i), false);
+        addAsmOperand(String.slice(Prev, i), IsIsolatedToken);
         InTok = false;
         IsIsolatedToken = false;
       }