feat: Reduce the number of parts created by tree shaking (#8472)

### Description

Extracted from #8420 to split diffs
for optimization vs intuitiveness

### Testing Instructions

---------

Co-authored-by: Tobias Koppers <[email protected]>

crates/turbopack-ecmascript/src/tree_shake/graph.rs

@@ -1124,6 +1124,13 @@ impl DepGraph {
             .map(|d| matches!(d, Dependency::Strong))
             .unwrap_or(false)
     }
+
+    pub(crate) fn has_path_connecting(&mut self, from: &ItemId, to: &ItemId) -> bool {
+        let from = self.g.node(from);
+        let to = self.g.node(to);
+
+        has_path_connecting(&self.g.idx_graph, from, to, None)
+    }
 }
 
 const ASSERT_CHUNK_KEY: &str = "__turbopack_part__";

crates/turbopack-ecmascript/src/tree_shake/mod.rs

@@ -182,6 +182,21 @@ impl Analyzer<'_> {
                     state
                         .last_writes
                         .retain(|last_write| !self.g.has_strong_dep(item_id, last_write));
+
+                    // Drop all writes which are not reachable from this item.
+                    //
+                    // For
+                    //
+                    // var x = 0
+                    // x = 1
+                    // x = 2
+                    // x += 3
+                    //
+                    // this will drop `x = 1` as not reachable from `x += 3`.
+
+                    state
+                        .last_writes
+                        .retain(|last_write| self.g.has_path_connecting(item_id, last_write));
                 }
 
                 // For each var in READ_VARS:

crates/turbopack-ecmascript/tests/tree-shaker/analyzer/grouping/input.js

@@ -0,0 +1,13 @@
+let x = 1;
+x = 2;
+x = 3;
+x = 4;
+x = 5;
+x += 6;
+x += 7;
+x += 8;
+x += 9;
+
+export { x };
+
+export const y = x;