R600: Move CONST_ADDRESS folding into AMDGPUDAGToDAGISel::Select()

This increases the number of opportunites we have for folding.  With the
previous implementation we were unable to fold into any instructions
other than the first when multiple instructions were selected from a
single SDNode.

Reviewed-by: Vincent Lejeune <vljn at ovi.com>
llvm-svn: 186919