sv: allow globals in one file to depend on globals in another

This defers the simplification of globals so that globals in one file
may depend on globals in other files. Adds a simplify() call downstream
because globals are appended at the end.

tests/verilog/parameters_across_files.ys

@@ -0,0 +1,20 @@
+read_verilog -sv <<EOF
+parameter Q = 1;
+EOF
+read_verilog -sv <<EOF
+parameter P = Q;
+module top(
+    output integer out
+);
+    assign out = P;
+    always @*
+        assert (out == 1);
+endmodule
+EOF
+
+hierarchy
+proc
+flatten
+opt -full
+select -module top
+sat -verify -seq 1 -tempinduct -prove-asserts -show-all