Fix splitting of assignments to SC variables (#6329) (#6336)

SC variables can only be assigned whole, so do not split up
concatenation assignments in V3FuncOpt.

Fixes #6329

src/V3FuncOpt.cpp

@@ -228,6 +228,8 @@ class FuncOptVisitor final : public VNVisitor {
         UASSERT_OBJ(lhsp->width() == rhsp->width(), nodep, "Inconsistent assignment");
         // Only consider pure assignments. Nodes inserted below are safe.
         if (!nodep->user1() && (!lhsp->isPure() || !rhsp->isPure())) return false;
+        // Do not split assignments to SC variables, they cannot be assigned in parts
+        if (lhsp->exists([](AstVarRef* refp) { return refp->varp()->isSc(); })) return false;
         // Check for a Sel on the LHS if present, and skip over it
         uint32_t lsb = 0;
         if (AstSel* const selp = VN_CAST(lhsp, Sel)) {