[llvm] [SandboxIR] Implement StoreInst (PR #99707)

[via llvm-commits]

vporpo wrote:

> A MemoryAccessBase for LoadInst and StoreInst with:

This is very reasonable, and it would be very useful in in code that works with either Loads or Stores. The only downside that I can think of is that it deviates from the LLVM class hierarchy, which we try to avoid as we want to make this API feel like LLVM as much as possible.

>  Instructions.h to split them out of SandboxIR.h

This is also very reasonable, and I think we should do this at some point in a followup NFC patch.

https://github.com/llvm/llvm-project/pull/99707