getset.h: S9xGetByte, S9xSetByte: Combine code common to all cases of the switch statements.

This code does not modify the timing of any SNES instruction.
getset.h: S9xGetWord, S9xSetWord: Combine code common to all cases of the switch statements, and combine memory access timings.

This code makes partial 16-bit writes impossible. In the presence of interrupts or events, this commit modifies the timing of any SNES 16-bit memory-load or memory-store instruction in the following ways:

* If an instruction would have modified the value visible on the SNES Open Bus before an interrupt or event, that interrupt or event may not see the value it expects.
* If an instruction would have modified one APU byte before an interrupt or event, the APU's reference timestamp will be delayed by one memory access.
* The emulation of other chips may react differently to partial writes being fulfilled entirely regardless of interrupts or events happening.