Remove rustc_middle::mir::tcx module.

This is a really weird module. For example, what does `tcx` in
`rustc_middle::mir::tcx::PlaceTy` mean? The answer is "not much".

The top-level module comment says:

> Methods for the various MIR types. These are intended for use after
> building is complete.

Awfully broad for a module that has a handful of impl blocks for some
MIR types, none of which really relates to `TyCtxt`. `git blame`
indicates the comment is ancient, from 2015, and made sense then.

This module is now vestigial. This commit removes it and moves all the
code within into `rustc_middle::mir::statement`. Some specifics:

- `Place`, `PlaceRef`, `Rvalue`, `Operand`, `BorrowKind`: they all have `impl`
  blocks in both the `tcx` and `statement` modules. The commit merges
  the former into the latter.

- `BinOp`, `UnOp`: they only have `impl` blocks in `tcx`. The commit
  moves these into `statement`.

- `PlaceTy`, `RvalueInitializationState`: they are defined in `tcx`.
  This commit moves them into `statement` *and* makes them available in
  `mir::*`, like many other MIR types.

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -1,7 +1,6 @@
 use std::mem;
 
 use rustc_index::IndexVec;
-use rustc_middle::mir::tcx::{PlaceTy, RvalueInitializationState};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::{bug, span_bug};

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -7,7 +7,6 @@ use rustc_data_structures::fx::{FxHashMap, FxIndexSet, StdEntry};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::DenseBitSet;
-use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt};