[AMDGPU][SIMemoryLegalizer] Combine all GFX6-9 CacheControl Classes

Merge the following classes into `SIGfx6CacheControl`:
- SIGfx7CacheControl
- SIGfx90ACacheControl
- SIGfx940CacheControl

They were all very similar and had a lot of duplicated boilerplate just to implement one or two codegen differences. GFX90A/GFX940 have a bit more differences, but they're still manageable under one class because the general behavior is the same.

This removes 500 lines of code and puts everything into a single place which I think makes it a lot easier to maintain, at the cost of a slight increase in complexity for some functions.

There is still a lot of room for improvement but I think this patch is already big enough as is and I don't want to bundle too much into one review.

Author: Pierre-vh