fix a really subtle bug in the cross section of aliases and TLS:

the SelectionDAG::getGlobalAddress function properly looks through
aliases to determine thread-localness, but then passes the GV* down
to GlobalAddressSDNode::GlobalAddressSDNode which does not.  Instead
of passing down isTarget, just pass down the predetermined node
opcode.  This fixes some assertions with out of tree changes I'm 
working on.

llvm-svn: 74325