Work in progress. This patch *fixes* x86-64 calls which are modelled as StructRet...

Work in progress. This patch *fixes* x86-64 calls which are modelled as StructRet but really should be return in registers, e.g. _Complex long double, some 128-bit aggregates. This is a short term solution that is necessary only because llvm, for now, cannot model i128 nor call's with multiple results.
Status: This only works for direct calls, and only the caller side is done. Disabled for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46527 91177308-0d34-0410-b5e6-96231b3b80d8