Handling of attributes for struct / union variables

(* nowrshmsk *) on a struct / union variable now affects dynamic
bit slice assignments to members of the struct / union.

(* nowrshmsk *) can in some cases yield significant resource savings; the
combination of pipeline shifting and indexed writes is an example of this.

Constructs similar to the one below can benefit from (* nowrshmsk *), and
in addition it is no longer necessary to split out the shift assignments
on separate lines in order to avoid the error message "ERROR: incompatible
mix of lookahead and non-lookahead IDs in LHS expression."

    always_ff @(posedge clk) begin
        if (rotate) begin
            { v5, v4, v3, v2, v1, v0 } <= { v4, v3, v2, v1, v0, v5 };

            if (res) begin
                v0.bytes <= '0;
            end else if (w) begin
                v0.bytes[addr] <= data;
            end
        end
    end

tests/svtypes/.gitignore

@@ -1,3 +1,4 @@
 /*.log
 /*.out
 /run-test.mk
+/temp

tests/svtypes/struct_dynamic_range.sv

@@ -4,7 +4,7 @@ module range_shift_mask(
     input logic [2:0] addr_o,
     output logic [7:0] data_o
 );
-    // (* nowrshmsk = 0 *)
+    (* nowrshmsk = 0 *)
     struct packed {
         logic [7:0] msb;
         logic [0:3][7:0] data;
@@ -24,7 +24,7 @@ module range_case(
     input logic [2:0] addr_o,
     output logic [7:0] data_o
 );
-    // (* nowrshmsk = 1 *)
+    (* nowrshmsk = 1 *)
     struct packed {
         logic [7:0] msb;
         logic [0:3][7:0] data;

tests/svtypes/struct_dynamic_range.ys

@@ -1,4 +1,8 @@
+! mkdir -p temp
 read_verilog -sv struct_dynamic_range.sv
+write_rtlil temp/struct_dynamic_range.il
+! grep -F -q ' cell $shift ' temp/struct_dynamic_range.il
+! grep -F -q ' switch $mul' temp/struct_dynamic_range.il
 prep -top top
 flatten
 sat -enable_undef -verify -prove-asserts