[RLE-DSE] Use SmallDenseMap instead of DenseMap in RLE to save memory allocations.

Many, if not most functions do not have more than 64 BasicBlocks (BBState)
which is the default for DenseMap.

Author: Xin Tong

lib/SILPasses/Scalar/RedundantLoadElimination.cpp

@@ -326,7 +326,7 @@ class RLEContext {
   llvm::DenseMap<MemLocation, unsigned> LocToBitIndex;
 
   /// A map from each BasicBlock to its BBState.
-  llvm::DenseMap<SILBasicBlock *, BBState> BBToLocState;
+  llvm::SmallDenseMap<SILBasicBlock *, BBState, 4> BBToLocState;
 
   /// A map for each basic block and whether its predecessors have forwardable
   /// edges.