[GlobalOpt] Fix support for casts in ctors.

Summary:
Fixing an issue where initializations of globals where constructors use
casts were silently translated to 0-initialization.

Reviewers: davidxl, evgeny777

Reviewed By: evgeny777

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45198

llvm-svn: 329409