[GlobalOpt] Collect initializer when building GlobalPart.

Update the code to collect the initializer when collecting the types for
a global, as suggested by @nikic in D144476.

This replaces code which bailed out if *any* part doesn't have an
initializer with a continue to just skip parts which don't have an
initializer.

Depends on D145489.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D145490