[MCA][LSUnit] Correctly update the internal group flags on store barrier execution. Fixes PR48024.

This is likely to be a regressigion introduced by my last refactoring of the
LSUnit (commit 5578ec32). Before this patch, the
"CurrentStoreBarrierGroupID" index was not correctly reset on store barrier
executions.  This was leading to unexpected crashes like the one reported as
PR48024.