[NFC] SIL: Expose undef values.

Author: nate-chandler

include/swift/SIL/SILFunction.h

@@ -28,6 +28,7 @@
 #include "swift/SIL/SILDeclRef.h"
 #include "swift/SIL/SILLinkage.h"
 #include "swift/SIL/SILPrintContext.h"
+#include "llvm/ADT/MapVector.h"
 
 namespace swift {
 
@@ -337,12 +338,8 @@ class SILFunction
 
   PerformanceConstraints perfConstraints = PerformanceConstraints::None;
 
-  /// This is the set of undef values we've created, for uniquing purposes.
-  ///
-  /// We use a SmallDenseMap since in most functions, we will have only one type
-  /// of undef if we have any at all. In that case, by staying small we avoid
-  /// needing a heap allocation.
-  llvm::SmallDenseMap<SILType, SILUndef *, 1> undefValues;
+  /// The undefs of each type in the function.
+  llvm::SmallMapVector<SILType, SILUndef *, 1> undefValues;
 
   /// This is the number of uses of this SILFunction inside the SIL.
   /// It does not include references from debug scopes.
@@ -1637,6 +1634,10 @@ class SILFunction
         decl->getLifetimeAnnotation());
   }
 
+  ArrayRef<std::pair<SILType, SILUndef *>> getUndefValues() {
+    return {undefValues.begin(), undefValues.end()};
+  }
+
   /// verify - Run the SIL verifier to make sure that the SILFunction follows
   /// invariants.
   void verify(CalleeCache *calleeCache = nullptr,