[CodeGen] Replace CCState's getNextStackOffset with getStackSize (NFC)

The term "next stack offset" is misleading because the next argument is
not necessarily allocated at this offset due to alignment constrains.
It also does not make much sense when allocating arguments at negative
offsets (introduced in a follow-up patch), because the returned offset
would be past the end of the next argument.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D149566