arch: Use const StaticInstPtr references where possible

This patch optimises the passing of StaticInstPtr by avoiding copying
the reference-counting pointer. This avoids first incrementing and
then decrementing the reference-counting pointer.

src/arch/sparc/stacktrace.hh

@@ -48,7 +48,7 @@ class StackTrace
 
   public:
     bool
-    trace(ThreadContext *tc, StaticInstPtr inst)
+    trace(ThreadContext *tc, const StaticInstPtr &inst)
     {
         panic("StackTrace::trace not implemented for SPARC.\n");
         return false;