Canonicalize on StartAppPC or DecodedPC, but not both.

[pgoodman]

DecodedBasicBlock uses StartAppPC, but has no associated EndAppPC. It also has a StartCachePC, but having an end is meaningless due to fragment scheduling.

NativeInstruction uses DecodedPC and DecodedLength. This could equivalently be represented with bounding PCs, using StartAppPC and EndAppPC.

I think we should canonicalize on these two function names across blocks and instructions.

It's also worth thinking about if Start or Begin is the right prefix.