avutil/x86util: don't use movss in VBROADCASTSS macro when src and dst args are the same

Reviewed-by: Henrik Gramner <henrik@gramner.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/x86/x86util.asm

@@ -833,7 +833,9 @@
 %if cpuflag(avx)
     vbroadcastss %1, %2
 %else ; sse
+%ifnidn %1, %2
     movss        %1, %2
+%endif
     shufps       %1, %1, 0
 %endif
 %endmacro