Move MirPass to rustc_mir_transform.

Because that's now the only crate that uses it.

Moving stuff out of `rustc_middle` is always welcome.

I chose to use `impl crate::MirPass`/`impl crate::MirLint` (with
explicit `crate::`) everywhere because that's the only mention of
`MirPass`/`MirLint` used in all of these files. (Prior to this change,
`MirPass` was mostly imported via `use rustc_middle::mir::*` items.)

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -27,7 +27,7 @@ impl InstSimplify {
     }
 }
 
-impl<'tcx> MirPass<'tcx> for InstSimplify {
+impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
     fn name(&self) -> &'static str {
         self.name()
     }