[WinEH] Ensure that funclets obey the x64 ABI

The x64 ABI requires that epilogues do not contain code other than stack
adjustments and some limited control flow.  However, we'd insert code to
initialize the return address after stack adjustments.  Instead, insert
EAX/RAX with the current value before we create the stack adjustments in
the epilogue.

llvm-svn: 248839