util: Implement unreachable for MSVC using __assume

Based on the description of __assume at:

http://msdn.microsoft.com/en-us/library/1b3fsfxw.aspx

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/util/macros.h

@@ -69,6 +69,12 @@ do {                        \
    assert(!str);            \
    __builtin_unreachable(); \
 } while (0)
+#elif _MSC_VER >= 1200
+#define unreachable(str)    \
+do {                        \
+   assert(!str);            \
+   __assume(0);             \
+} while (0)
 #endif
 
 #ifndef unreachable