Move LHS type casts to RHS in assign_* instructions

Doing so during goto-program conversion will ensure we can generate appropriate
conversion checks.

Fixes: #4208

Author: tautschnig

regression/cbmc/Overflow_Addition3/test.desc

@@ -1,4 +1,4 @@
-KNOWNBUG
+CORE
 main.c
 --signed-overflow-check --conversion-check
 ^EXIT=10$

src/goto-programs/goto_convert_side_effect.cpp

@@ -107,7 +107,11 @@ void goto_convertt::remove_assignment(
     rhs.type() = expr.op0().type();
     rhs.add_source_location() = expr.source_location();
 
-    code_assignt assignment(expr.op0(), rhs);
+    exprt new_lhs = skip_typecast(expr.op0());
+    rhs = typecast_exprt::conditional_cast(rhs, new_lhs.type());
+    rhs.add_source_location() = expr.source_location();
+
+    code_assignt assignment(new_lhs, rhs);
     assignment.add_source_location()=expr.source_location();
 
     convert(assignment, dest, mode);