Add VPX_SWAP macro

Change-Id: I60e233eddef238ad918183392794084673f27d2d

vpx_dsp/vpx_dsp_common.h

@@ -46,6 +46,13 @@ extern "C" {
 # define UNLIKELY(v)  (v)
 #endif
 
+#define VPX_SWAP(type, a, b) \
+  do {                       \
+    type c = (b);            \
+    b = a;                   \
+    a = c;                   \
+  } while (0)
+
 #if CONFIG_VP9_HIGHBITDEPTH
 // Note:
 // tran_low_t  is the datatype used for final transform coefficients.