by having X86DAGToDAGISel::SelectAddr get passed in the parent node
of the operand match (the load/store/atomic op) and having it get
the address space from that, instead of having special FS/GS addr
mode operations that require duplicating the entire instruction set
to support.

This makes FS and GS relative accesses *far* more predictable and
work much better.  It also simplifies the X86 backend a bit, more
to come.

There is still a pending issue with nodes like ISD::PREFETCH and
X86ISD::FLD, which really should be MemSDNode's but aren't.

llvm-svn: 114491