Simplify and optimize value extractors

Speed up results by Joona:

gcc-4.7 (1.5%)
gcc-4.8 (0.5%)
gcc-4.9 (1.0%)

Speed up results by mstembera:

gcc 474
p-value: 0.719

gcc 482
p-value: 1

gcc 492
p-value: 0.859

No functional change

Resolves #211

src/types.h

@@ -273,11 +273,15 @@ inline Score make_score(int mg, int eg) {
 /// according to the standard a simple cast to short is implementation defined
 /// and so is a right shift of a signed integer.
 inline Value mg_value(Score s) {
-  return Value(((s + 0x8000) & ~0xFFFF) / 0x10000);
+
+  union { uint16_t u; int16_t s; } mg = { uint16_t(unsigned(s + 0x8000) >> 16) };
+  return Value(mg.s);
 }
 
 inline Value eg_value(Score s) {
-  return Value((int)(unsigned(s) & 0x7FFFU) - (int)(unsigned(s) & 0x8000U));
+
+  union { uint16_t u; int16_t s; } eg = { uint16_t(unsigned(s)) };
+  return Value(eg.s);
 }
 
 #define ENABLE_BASE_OPERATORS_ON(T)                             \