MCAssembler tweak for determining when a symbol difference is resolved.

MCAssembler tweak for determining when a symbol difference is resolved.

If the two fragments are in the same Atom, then the difference
expression is resolvable at compile time. Previously we were checking
that they were in the same fragment, but that breaks down in the
presence of instruction relaxation which has multiple fragments in the
same atom.

rdar://10711829

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148423 91177308-0d34-0410-b5e6-96231b3b80d8