Fix select from non-variable-reference error (pr2281519)

VHDL can't select bits from arbitrary expression so sometimes
translating IVL_EX_SELECT would fail. This is easily fixed by
replacing the select with a shift in this instance (and the
resizing)

tgt-vhdl/expr.cc

@@ -451,20 +451,26 @@ static vhdl_expr *translate_select(ivl_expr_t e)
       return NULL;
 
    ivl_expr_t o2 = ivl_expr_oper2(e);
-   if (o2) {
-      vhdl_var_ref *from_var_ref = dynamic_cast<vhdl_var_ref*>(from);
-      if (NULL == from_var_ref) {
-         error("Can only select from variable reference");
-         return NULL;
-      }
-
+   if (o2) {      
       vhdl_expr *base = translate_expr(ivl_expr_oper2(e));
       if (NULL == base)
          return NULL;
 
-      vhdl_type integer(VHDL_TYPE_INTEGER);
-      from_var_ref->set_slice(base->cast(&integer), ivl_expr_width(e) - 1);
-      return from_var_ref;
+      vhdl_var_ref *from_var_ref = dynamic_cast<vhdl_var_ref*>(from);
+      if (NULL == from_var_ref) {
+         // We can't directly select bits from something that's not
+         // a variable reference in VHDL, but we can emulate the
+         // effect with a shift and a resize
+         return new vhdl_binop_expr(from, VHDL_BINOP_SR, base->to_integer(),
+                                    new vhdl_type(*from->get_type()));
+      }
+      else {
+         // We can use the more idomatic VHDL slice notation on a
+         // single variable reference
+         vhdl_type integer(VHDL_TYPE_INTEGER);
+         from_var_ref->set_slice(base->cast(&integer), ivl_expr_width(e) - 1);
+         return from_var_ref;
+      }
    }
    else
       return from->resize(ivl_expr_width(e));
@@ -634,7 +640,7 @@ vhdl_expr *translate_expr(ivl_expr_t e)
 {
    assert(e);
    ivl_expr_type_t type = ivl_expr_type(e);
-   
+
    switch (type) {
    case IVL_EX_STRING:
       return translate_string(e);