R600: Move computeMaskedBitsForTargetNode out of AMDILISelLowering.cpp

Remove handling of select_cc, since it makes no sense to be there. This
now does nothing, but I'll be adding some handling of other target nodes
soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204743 91177308-0d34-0410-b5e6-96231b3b80d8