fix Issue 11265 - Segfault while calling instance method of class defined inside struct

The regression was introduced by the commit:
https://github.com/D-Programming-Language/dmd/commit/671b7c20b96b580ee1dc2b3f5e3ee66c63d079b6#diff-43282ebf5a2de5fdbcb3b5083ddf949dR147

vtbl[] calculation is essentially unrelated to the class instance size.
So, just only once initializing of vtbl[] and running semantic on member
functions is sufficient.