Struct byval cleanup: add helper functions to reduce code duplication.

Helper functions are added:
emitPostLd: emit a post-increment load operation with given size.
emitPostSt: emit a post-increment store operation with given size.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193656 91177308-0d34-0410-b5e6-96231b3b80d8