Have TableGen emit setSubgraphColor calls under control of a -gen-debug

Have TableGen emit setSubgraphColor calls under control of a -gen-debug
flag.  Then in a debugger developers can set breakpoints at these calls
to see waht is about to be selected and what the resulting subgraph
looks like.  This really helps when debugging instruction selection.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58278 91177308-0d34-0410-b5e6-96231b3b80d8