Fix PR8565.

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