avutil/x86/bswap: Add msvc bswap instrinsics.

This adds msvc optimisations as well as fixing an error in icl whereby it will generate invalid code otherwise.
Signed-off-by: Matt Oliver <protogonoi@gmail.com>

libavutil/x86/bswap.h

@@ -28,7 +28,30 @@
 #include "config.h"
 #include "libavutil/attributes.h"
 
-#if HAVE_INLINE_ASM
+#if defined(_MSC_VER)
+
+#define av_bswap16 av_bswap16
+static av_always_inline av_const uint16_t av_bswap16(uint16_t x)
+{
+    return _rotr16(x, 8);
+}
+
+#define av_bswap32 av_bswap32
+static av_always_inline av_const uint32_t av_bswap32(uint32_t x)
+{
+    return _byteswap_ulong(x);
+}
+
+#if ARCH_X86_64
+#define av_bswap64 av_bswap64
+static inline uint64_t av_const av_bswap64(uint64_t x)
+{
+    return _byteswap_uint64(x);
+}
+#endif
+
+
+#elif HAVE_INLINE_ASM
 
 #if AV_GCC_VERSION_AT_MOST(4,0)
 #define av_bswap16 av_bswap16