Add WEBP_RESTRICT & use it in VP8BitReader

Marking the `VP8BitReader` as `__restrict__` helps the compiler generate
better code avoiding issues related to aliasing (re-loads/stores).

Change-Id: Ib7178f57e27e5f40572efc3e567cdf994ea6d928

src/utils/bit_reader_inl_utils.h

@@ -104,7 +104,7 @@ void VP8LoadNewBytes(VP8BitReader* const br) {
 }
 
 // Read a bit with proba 'prob'. Speed-critical function!
-static WEBP_INLINE int VP8GetBit(VP8BitReader* const br,
+static WEBP_INLINE int VP8GetBit(VP8BitReader* WEBP_RESTRICT const br,
                                  int prob, const char label[]) {
   // Don't move this declaration! It makes a big speed difference to store
   // 'range' *before* calling VP8LoadNewBytes(), even if this function doesn't

src/utils/utils.h

@@ -25,6 +25,16 @@
 #include "src/dsp/dsp.h"
 #include "src/webp/types.h"
 
+//------------------------------------------------------------------------------
+// restrict
+
+#if defined(__GNUC__) || defined(__clang__)
+#define WEBP_RESTRICT __restrict
+#else
+#define WEBP_RESTRICT
+#endif
+
+
 #ifdef __cplusplus
 extern "C" {
 #endif