alpha: make hwrei a control inst

this always changes the PC and is basically an impromptu branch instruction. why
not speculate on this instead of always be forced to mispredict/squash after the
hwrei gets resolved?

The InOrder model needs this marked as "isControl" so it knows to update the PC
after the ALU executes it. If this isnt marked as control, then it's going to
force the model to check the PC of every instruction at commit (what O3 does?),
and that would be a wasteful check for a very high percentage of instructions.

src/arch/alpha/isa/decoder.isa

@@ -935,7 +935,7 @@ decode OPCODE default Unknown::unknown() {
   0x1e: decode PALMODE {
       0: OpcdecFault::hw_rei();
         format BasicOperate {
-          1: hw_rei({{ xc->hwrei(); }}, IsSerializing, IsSerializeBefore);
+          1: hw_rei({{ xc->hwrei(); }}, IsSerializing, IsSerializeBefore, IsControl);
         }
     }