glsl: don't drop instructions from unreachable terminators continue branch

glsl: don't drop instructions from unreachable terminators continue branch

These instructions will be executed on every iteration of the loop
we cannot drop them.

V2:
- move removal of unreachable terminators from the terminator list
  to the same place they are removed from the IR as suggested by
  Nicolai.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>