DAG: propagate ConsecutiveRegs flags to returns too.

Arguments already have a flag to inform backends when they have been split up.
The AArch64 arm64_32 ABI makes use of these on return types too, so that code
emitted for armv7k can be ABI-compliant.

There should be no CodeGen changes yet, just making more information available.

llvm-svn: 358399