fix width detection of array querying function in case and case item expressions

I also removed the unnecessary shadowing of `width_hint` and `sign_hint`
in the corresponding case in `simplify()`.

tests/simple/case_expr_extend.sv

@@ -0,0 +1,11 @@
+module top(
+    output logic [5:0] out
+);
+always_comb begin
+    out = '0;
+    case (1'b1 << 1)
+        2'b10: out = '1;
+        default: out = '0;
+    endcase
+end
+endmodule