Move MirPatch from rustc_middle to rustc_mir_transform.

Because it's only used in `rustc_mir_transform`. (Presumably it is
currently in `rustc_middle` because lots of other MIR-related stuff is,
but that's not a hard requirement.) And because `rustc_middle` is huge
and it's always good to make it smaller.

compiler/rustc_mir_transform/src/unreachable_prop.rs

@@ -6,10 +6,11 @@ use rustc_abi::Size;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::Scalar;
-use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, TyCtxt};
 
+use crate::patch::MirPatch;
+
 pub(super) struct UnreachablePropagation;
 
 impl crate::MirPass<'_> for UnreachablePropagation {