Extend the (abi-irrelevant) return convention to be able to return more than two...

Extend the (abi-irrelevant) return convention to be able to return more than two values in
integer registers.  This is already supported by the fastcc convention, but it doesn't
hurt to support it in the standard conventions as well.

In cases where we can cheat at the calling convention, this allows us to avoid returning
things through memory in more cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157698 91177308-0d34-0410-b5e6-96231b3b80d8