LLVMContext: Store APInt/APFloat directly into the ConstantInt/FP DenseMaps.

Required some APInt massaging to get proper empty/tombstone values. Apart
from making the code a bit simpler this also reduces the bucket size of
the ConstantInt map from 32 to 24 bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223478 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/APInt.h

@@ -91,6 +91,8 @@ class APInt {
     APINT_WORD_SIZE = static_cast<unsigned int>(sizeof(uint64_t))
   };
 
+  friend struct DenseMapAPIntKeyInfo;
+
   /// \brief Fast internal constructor
   ///
   /// This constructor is used only internally for speed of construction of
@@ -277,7 +279,6 @@ public:
   /// Simply makes *this a copy of that.
   /// @brief Copy Constructor.
   APInt(const APInt &that) : BitWidth(that.BitWidth), VAL(0) {
-    assert(BitWidth && "bitwidth too small");
     if (isSingleWord())
       VAL = that.VAL;
     else