Set the lvIsMultiRegArgOrRet for a variable containing the result of

Set the lvIsMultiRegArgOrRet for a variable containing the result of
inlined multi-register return call.

This change makes sure the lvIsMultiRegArgOrRet on a variable used to
store the result of inlined multi-register return function is set.
The way the code works today, this is not an issue since the code in
fgAttachStructInlineeToAsg always uses a CopyObj/Blk to asign the return
value from the registers to the var on stack. The CopyBlock/Obj gets the
address of the variable making it address-exposed and that prevents struct
promotion.
In the future (when CopyObj/Blk is not used) not having
lvIsMultiRegArgOrRet set could cause a problem.

Fixes issue 4276.

Commit migrated from https://github.com/dotnet/coreclr/commit/d07a34f33a862a40a1456cbff245a1e141a7264d