Fix MULTIDRIVENPROC on static loop induction variables (#8242)

Disable MULTIDRIVENPROC for static variables used as loop induction
variables. Same way as we disable similar warning in Dfg. While bad
style and bad for performance, it's common legacy code style.

src/V3Undriven.cpp

@@ -691,7 +691,7 @@ class UndrivenVisitor final : public VNVisitorConst {
                     }
                     warnClockingDriven(nodep, entryp, otherWritep, otherWriteIsStaticInit);
                 }
-                if (multidrivenCommon
+                if (multidrivenCommon && !nodep->varp()->isUsedLoopIdx()
                     && !nodep->varp()->fileline()->warnIsOff(V3ErrorCode::MULTIDRIVENPROC)) {
                     // Two plain always blocks driving the whole signal: legal
                     // SystemVerilog, but a driver conflict for synthesis. The