Rollup merge of #114505 - ouz-a:cleanup_mir, r=RalfJung

Add documentation to has_deref

Documentation of `has_deref` needed some polish to be more clear about where it should be used and what's it's purpose.

cc https://github.com/rust-lang/rust/issues/114401

r? `@RalfJung`

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -839,7 +839,7 @@ impl Map {
         tail_elem: Option<TrackElem>,
         f: &mut impl FnMut(ValueIndex),
     ) {
-        if place.has_deref() {
+        if place.is_indirect_first_projection() {
             // We do not track indirect places.
             return;
         }