sv: fix size cast clipping expression width

tests/simple/lesser_size_cast.sv

@@ -0,0 +1,7 @@
+module top (
+    input signed [1:0] a,
+    input signed [2:0] b,
+    output signed [4:0] c
+);
+    assign c = 2'(a) * b;
+endmodule