[RLE-DSE] Implement the actual redundant load elimination (RLE)

data flow. This should handle some simple RLE. but not phi-node, i.e.
SILArguments.

Also, the new RLE implemented in this patch is disabled by default (behind
a flag). we are working on post-commit review.

include/swift/SIL/MemLocation.h

@@ -39,6 +39,7 @@ class MemLocation;
 class LoadStoreValue;
 using LoadStoreValueList = llvm::SmallVector<LoadStoreValue, 8>;
 using MemLocationValueMap = llvm::DenseMap<MemLocation, LoadStoreValue>; 
+using ValueTableMap = llvm::SmallMapVector<unsigned, LoadStoreValue, 8>;
 
 /// This class represents either a single SILValue or a covering of values that
 /// we can forward from via the introdution of a SILArgument. This enables us
@@ -152,6 +153,13 @@ class LoadStoreValue {
     return *this;
   }
 
+  /// Returns whether the LoadStoreValue has been initialized properly.
+  bool isValid() const {
+    if (IsCoveringValue)
+      return true;
+    return Base && Path.hasValue();
+  }
+
   /// Returns true if the LoadStoreValue has a non-empty projection path.
   bool hasEmptyProjectionPath() const { return !Path.getValue().size(); }