Don't use EmitAbsValue with symbol references.

The function exists to force an expression to be absolute, but there it is not
possible to force a symbol reference since

a = b
.long a

means something else.

This is an alternative fix for pr9951 that uses an assert. It then deletes
the old pr9951 test that was testing nothing already.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204399 91177308-0d34-0410-b5e6-96231b3b80d8