[NFC] Add a lazy mode to LocalGraph (#6895)

LocalGraph by default will compute all the local.sets that can be read from all
local.gets. However, many passes only query a small amount of those. To
avoid wasted work, add a lazy mode that only computes sets when asked about
a get.

This is then used in a single place, LoopInvariantCodeMotion, which becomes
18% faster.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/passes/LoopInvariantCodeMotion.cpp b/src/passes/LoopInvariantCodeMotion.cpp
index 321267e8e..1277c1a34 100644
--- a/src/passes/LoopInvariantCodeMotion.cpp
+++ b/src/passes/LoopInvariantCodeMotion.cpp
@@ -45,11 +45,12 @@ struct LoopInvariantCodeMotion
 
   // main entry point
 
-  LocalGraph* localGraph;
+  LazyLocalGraph* localGraph;
 
   void doWalkFunction(Function* func) {
-    // Compute all local dependencies first.
-    LocalGraph localGraphInstance(func, getModule());
+    // Prepare to compute the local dependencies we care about. We may only need
+    // very few, so use a lazy LocalGraph.
+    LazyLocalGraph localGraphInstance(func, getModule());
     localGraph = &localGraphInstance;
     // Traverse the function.
     super::doWalkFunction(func);