Some is_locked checks are unnecessary

[skoppe]

Due to a small semantic change in porting the C++ code over to D we are calling is_locked more than we need to.

D didn't have compare_exchange_weak so I used casWeak. The difference is that compare_exchange_weak also loads the value if the compare fails, whereas casWeak only compares and doesn't load the value.

That means that these checks

concurrency/source/concurrency/stoptoken.d

Line 311 in f5e280f

while (is_locked(oldState)) {

concurrency/source/concurrency/stoptoken.d

Line 326 in f5e280f

while (is_locked(oldState)) {

could be skipped (they are always false) AFTER the casWeak in the outer while failed.