[APInt] Use MathExtras.h BitsToFloat/Double and Float/DoubleToBits instead of type punning through a union

[craig.topper]

Summary

The functions in MathExtras.h uses a safer memcpy instead of going through a union.

[RKSimon]

LGTM

include/llvm/ADT/APInt.h

   double signedRoundToDouble() const { return roundToDouble(true); }
 
   /// \brief Converts APInt bits to a double
   ///
   /// The conversion does not do a translation from integer to double, it just
   /// re-interprets the bits as a double. Note that it is valid to do this on
   /// any bit width. Exactly 64 bits will be translated.
   double bitsToDouble() const {
-    union {
-      uint64_t I;
-      double D;
-    } T;
-    T.I = (isSingleWord() ? U.VAL : U.pVal[0]);
-    return T.D;
+    return BitsToDouble(getWord(0));
   }
 
   /// \brief Converts APInt bits to a double
   ///
   /// The conversion does not do a translation from integer to float, it just
   /// re-interprets the bits as a float. Note that it is valid to do this on
   /// any bit width. Exactly 32 bits will be translated.
   float bitsToFloat() const {
-    union {
-      unsigned I;
-      float F;
-    } T;
-    T.I = unsigned((isSingleWord() ? U.VAL : U.pVal[0]));
-    return T.F;
+    return BitsToFloat(getWord(0));
   }
 
   /// \brief Converts a double to APInt bits.
   ///
   /// The conversion does not do a translation from double to integer, it just
   /// re-interprets the bits of the double.
   static APInt doubleToBits(double V) {
-    union {
-      uint64_t I;
-      double D;
-    } T;
-    T.D = V;
-    return APInt(sizeof T * CHAR_BIT, T.I);
+    return APInt(sizeof(double) * CHAR_BIT, DoubleToBits(V));
   }
 
   /// \brief Converts a float to APInt bits.
   ///
   /// The conversion does not do a translation from float to integer, it just
   /// re-interprets the bits of the float.
   static APInt floatToBits(float V) {
-    union {
-      unsigned I;
-      float F;
-    } T;
-    T.F = V;
-    return APInt(sizeof T * CHAR_BIT, T.I);
+    return APInt(sizeof(float) * CHAR_BIT, FloatToBits(V));
   }
 
   /// @}
   /// \name Mathematics Operations
   /// @{
 
   /// \returns the floor log base 2 of this APInt.
   unsigned logBase2() const { return BitWidth - 1 - countLeadingZeros(); }