[rbi] Add translateSILAssignIndirect helpers for single-destination indirect parameter assigns
Add two template overloads of translateSILAssignIndirect that wrap
translateSILMultiAssign for the common case of assigning to a single
indirect parameter destination:
1. A generic template accepting any collection of source operands
2. A specialization for a single source operand
These simplify call sites that only need to handle one destination.