Add X86FastISel support for return statements. This entails refactoring
a bunch of stuff, to allow the target-independent calling convention
logic to be employed.

llvm-svn: 107800

Split the SDValue out of OutputArg so that SelectionDAG-independent
code can do calling-convention queries. This obviates OutputArgReg.

llvm-svn: 107786

Move CallingConvLower.cpp out of the SelectionDAG directory.

llvm-svn: 107781