[PATCH] D100744: Use SmallVector instead of std::vector to manage storage of llvm::BitVector

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This is a follow-up to https://reviews.llvm.org/D100387.
`std::vector` is not the best storage container here. My local benchmark (counting the number of instruction when compiling the sqlite3 amalgametion) yields the following:

- `std::vector<BitVector>` -> 5,860,885,896
- `SmallVector<BitWord, 0>` -> 5,858,991,997
- `SmallVector<BitWord> -> 5,817,679,224


https://reviews.llvm.org/D100744

Files:
  llvm/include/llvm/ADT/BitVector.h


Index: llvm/include/llvm/ADT/BitVector.h
===================================================================
--- llvm/include/llvm/ADT/BitVector.h
+++ llvm/include/llvm/ADT/BitVector.h
@@ -79,7 +79,7 @@
   static_assert(BITWORD_SIZE == 64 || BITWORD_SIZE == 32,
                 "Unsupported word size");
 
-  using Storage = std::vector<BitWord>;
+  using Storage = SmallVector<BitWord>;
 
   Storage Bits;  // Actual bits.
   unsigned Size; // Size of bitvector in bits.


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