Define CallSDNode, an SDNode subclass for use with ISD::CALL.

Define CallSDNode, an SDNode subclass for use with ISD::CALL.
Currently it just holds the calling convention and flags
for isVarArgs and isTailCall.

And it has several utility methods, which eliminate magic
5+2*i and similar index computations in several places.

CallSDNodes are not CSE'd. Teach UpdateNodeOperands to handle
nodes that are not CSE'd gracefully.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56183 91177308-0d34-0410-b5e6-96231b3b80d8