[CIR][Lowering] fixes return value for = operator for bitfields

[gitoleg]

Basically that is - the return value for = operator for bitfield assignment is wrong now. For example, the next function returns 7 for 3 bit bit field, though it should be -1:

int get_a(T *t) {
  return (t->a = 7);
}

This PR fix it. Actually, the bug was in the lowering - the integer cast is applied in the wrong place (in comparison with the original codegen).

[keryell]

Nice catch!

[bcardosolopes]

Minor conflict because of your previous merged PR, but LGTM otherwise

[gitoleg]

updated!