Allow usage of walrus operator in unconditional subexpressions

[Sainan]

Fixes #815, fixes #816. Doesn't seem to make any regressions with regards to #776.

[Sainan]

With the and case it's a bit tricky, because this UB is still possible:

if false and a := 1 then
else
    print(a)
end

[XmiliaH]

This seems to be bugged:

local x = 2

while (x+2+a:=2) do
    print(a)
end

should print 2 but prints 4.