Rewrite "if (Var) ASSERT(FALSE);" as "ASSERT(!Var);"

"if (Var) ASSERT(FALSE);" is better written as "ASSERT(!Var);":
easier to read code, more explicit report when triggered, ....

if.*ASSERT(FALSE) grep

NB:
Scope of this issue is explicitly limited.
Further similar improvements would be done in follow-up issue(s).

PS:
I will attach one (individual) patch at a time, to ease review. (As they are unrelated to one another.)