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/alpha/stacktrace.cc

@@ -122,7 +122,7 @@ StackTrace::StackTrace()
 {
 }
 
-StackTrace::StackTrace(ThreadContext *_tc, StaticInstPtr inst)
+StackTrace::StackTrace(ThreadContext *_tc, const StaticInstPtr &inst)
     : tc(0), stack(64)
 {
     trace(_tc, inst);