Three changes:

1. Introduce some enums and accessors in the InlineAsm class
   that eliminate a ton of magic numbers when handling inline
   asm SDNode.
2. Add a new MDNodeSDNode selection dag node type that holds
   a MDNode (shocking!)
3. Add a new argument to ISD::INLINEASM nodes that hold !srcloc
   metadata, propagating it to the instruction emitter, which
   drops it.

No functionality change.

llvm-svn: 100605