Rollup merge of #154297 - RalfJung:mir-opt-comments, r=dianqk

fix/extend some mir-opt comments

Looks like CopyProp was refactored without updating that comment.

And for GVN, I think this is what you had in mind @cjgillot but would be great if you could have a look.

compiler/rustc_mir_transform/src/copy_prop.rs

diff --git a/compiler/rustc_mir_transform/src/copy_prop.rs b/compiler/rustc_mir_transform/src/copy_prop.rs
index 89ee963..e04cb26 100644
--- a/compiler/rustc_mir_transform/src/copy_prop.rs
+++ b/compiler/rustc_mir_transform/src/copy_prop.rs
@@ -58,7 +58,8 @@ fn is_required(&self) -> bool {
     }
 }
 
-/// Utility to help performing substitution of `*pattern` by `target`.
+/// Utility to help performing substitution: for all key-value pairs in `copy_classes`,
+/// all occurrences of the key get replaced by the value.
 struct Replacer<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     unified: DenseBitSet<Local>,

compiler/rustc_mir_transform/src/gvn.rs

diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 245ee6e..e9a20aa 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -48,8 +48,15 @@
 //! # Handling of references
 //!
 //! We handle references by assigning a different "provenance" index to each Ref/RawPtr rvalue.
-//! This ensure that we do not spuriously merge borrows that should not be merged. Meanwhile, we
-//! consider all the derefs of an immutable reference to a freeze type to give the same value:
+//! This ensure that we do not spuriously merge borrows that should not be merged. For instance:
+//! ```ignore (MIR)
+//! _x = &_a;
+//! _a = 0;
+//! _y = &_a; // cannot be turned into `_y = _x`!
+//! ```
+//!
+//! On top of that, we consider all the derefs of an immutable reference to a freeze type to give
+//! the same value:
 //! ```ignore (MIR)
 //! _a = *_b // _b is &Freeze
 //! _c = *_b // replaced by _c = _a