[PATCH] Introduce bitset metadata format and bitset lowering pass.

[Kostya Serebryany]

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:175
@@ +174,3 @@
+
+  Int1Ty = Type::getInt1Ty(M.getContext());
+  Int8Ty = Type::getInt8Ty(M.getContext());
----------------
You can get all of these, except for IntPtrTy, from the IRBuilder. Just FYI, this way is also fine, I think. 

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:189
@@ +188,3 @@
+
+BitSetInfo LowerBitSets::buildBitSet(
+    MDString *BitSet,
----------------
I would add comments for every non-trivial function.
In most cases, I understand what these things are doing, other readers (including myself in 6 months) will probably be puzzled. 

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:292
@@ +291,3 @@
+  Value *Bit;
+  if (BSI.Bits.size() <= 8) {
+    // If the bit set is sufficiently small, we can avoid a load by bit testing
----------------
split into a separate function?

http://reviews.llvm.org/D7288

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