ScopInfo: simplify equivalence classes before storing them [NFCI]

This change has no impact on upstream Polly directly, but reduces output
noise for some internal isl versions we are testing. In general, storing
simpler and more canonical output is a good idea. Hence, it seems useful
to upstream this change.

llvm-svn: 336281
diff --git a/polly/lib/Analysis/ScopInfo.cpp b/polly/lib/Analysis/ScopInfo.cpp
index 5c46c76..b945fa6 100644
--- a/polly/lib/Analysis/ScopInfo.cpp
+++ b/polly/lib/Analysis/ScopInfo.cpp
@@ -3757,6 +3757,8 @@
     if (Consolidated)
       continue;
 
+    MACtx = MACtx.coalesce();
+
     // If we did not consolidate MA, thus did not find an equivalence class
     // for it, we create a new one.
     InvariantEquivClasses.emplace_back(