This is the first of a series of patches that adds caller support for by-value arguments that are passed in a single GPR.
There are 3 limitation cases:
-The by-value argument is larger than a single register.
-There are no remaining GPRs even though the by-value argument would otherwise fit in a single GPR.
-The by-value argument requires alignment greater than register width.
Future patches will be required to add support for these cases as well as for the callee handling (in LowerFormalArguments_AIX) that corresponds to this work.