Fix PR15293: ARM codegen ice - expected larger existing stack allocation

[Stepan Dyatkovskiy]

> About refactoring. I propose do it in two stages, first rename
> "getVarArgsRegSaveSize" to "get/setByValRegsSaveSize", and detach
> "StoreByValRegs" functionality from "VarArgStyleRegisters" method.
Sorry for typos, I mean:
First one is rename patch, second one is "StoreByValRegs" patch.

-Stepan.