verilog: Fix const eval of unbased unsized constants

When the verilog frontend perfomed constant evaluation of unbased
unsized constants in a context-determined expression it did not properly
extend them by repeating the bit value. This only affected constant
evaluation and not constants that made it through unchanged to RTLIL.
The latter case was already covered by tests and working before.

This fixes the const-eval issue by checking the `is_unsized` flag in
bitsAsConst and extending the value accordingly.

The newly added test also tests the already working non-const-eval case
to highlight that both cases should behave the same.

frontends/ast/ast.cc

@@ -847,7 +847,7 @@ RTLIL::Const AstNode::bitsAsConst(int width, bool is_signed)
 		bits.resize(width);
 	if (width >= 0 && width > int(bits.size())) {
 		RTLIL::State extbit = RTLIL::State::S0;
-		if (is_signed && !bits.empty())
+		if ((is_signed || is_unsized) && !bits.empty())
 			extbit = bits.back();
 		while (width > int(bits.size()))
 			bits.push_back(extbit);

tests/verilog/unbased_unsized_shift.sv

@@ -0,0 +1,28 @@
+module pass_through(
+    input [63:0] inp,
+    output [63:0] out
+);
+    assign out = inp;
+endmodule
+
+module top;
+    logic [63:0] s0c, s1c, sxc, s0d, s1d, sxd, d;
+
+    pass_through pt(8, d);
+
+    assign s0c = '0 << 8;
+    assign s1c = '1 << 8;
+    assign sxc = 'x << 8;
+    assign s0d = '0 << d;
+    assign s1d = '1 << d;
+    assign sxd = 'x << d;
+
+    always @* begin
+        assert (s0c === 64'h0000_0000_0000_0000);
+        assert (s1c === 64'hFFFF_FFFF_FFFF_FF00);
+        assert (sxc === 64'hxxxx_xxxx_xxxx_xx00);
+        assert (s0d === 64'h0000_0000_0000_0000);
+        assert (s1d === 64'hFFFF_FFFF_FFFF_FF00);
+        assert (sxd === 64'hxxxx_xxxx_xxxx_xx00);
+    end
+endmodule

tests/verilog/unbased_unsized_shift.ys

@@ -0,0 +1,7 @@
+read_verilog -sv unbased_unsized_shift.sv
+hierarchy
+proc
+flatten
+opt -full
+select -module top
+sat -verify -seq 1 -tempinduct -prove-asserts -show-all