[llvm] [RegAllocFast] Document the allocator state and operand phases. NFC (PR #219835)

[Nikhil Kotikalapudi via llvm-commits]

https://github.com/nkotikal commented:

Some of the changes make sense and help, but I'm a bit concerned about scope here, as the changes seem a bit scattered (especially removing the P.PhysReg local variable). 

I do agree that it's redundant but doesn't seem like it follows this PR's overall documentation theme. 

https://github.com/llvm/llvm-project/pull/219835