cpu: Fix InvalidRegClass access in CheckerCPU

This is fixing long regressions:

https://jenkins.gem5.org/job/nightly/252/console

Change-Id: Ica56b950e5091d3357060f566e02195cc8136924
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60449
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

src/cpu/checker/cpu.hh

@@ -176,7 +176,10 @@ class CheckerCPU : public BaseCPU, public ExecContext
     RegVal
     getRegOperand(const StaticInst *si, int idx) override
     {
-        return thread->getReg(si->srcRegIdx(idx));
+        const RegId& id = si->srcRegIdx(idx);
+        if (id.is(InvalidRegClass))
+            return 0;
+        return thread->getReg(id);
     }
 
     void