Fix PR8565.

This moves most of the isUsed logic to the MCSymbol itself. With this we
get a bit more relaxed about allowing definitions after uses: uses that
don't evaluate their argument immediately (jmp foo) are accepted.

ddunbar, this was the smallest compromise I could think of that lets us
accept gcc (and clang!) assembly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119144 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCSymbol.cpp

@@ -40,6 +40,7 @@ static bool NameNeedsQuoting(StringRef Str) {
 }
 
 void MCSymbol::setVariableValue(const MCExpr *Value) {
+  assert(!IsUsed && "Cannot set a variable that has already been used.");
   assert(Value && "Invalid variable value!");
   assert((isUndefined() || (isAbsolute() && isa<MCConstantExpr>(Value))) &&
          "Invalid redefinition!");