IRGen: Exact casts cannot fail

[aschwaighofer]

Use initialized metadata in exact checks otherwise we will fail in
[exact] checks when we shouldn't.

rdar://38885852

[aschwaighofer]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 260594c

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 260594c

[aschwaighofer]

Oh, we don't seem to support testing this branch.

09:19:39 Traceback (most recent call last):
09:19:39   File "/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch-03-26-2018/swift/utils/update-checkout", line 15, in <module>
09:19:39     update_checkout.main()
09:19:39   File "/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch-03-26-2018/swift/utils/update_checkout.py", line 481, in main
09:19:39     skip_repo_list)
09:19:39   File "/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch-03-26-2018/swift/utils/update_checkout.py", line 292, in obtain_all_additional_swift_sources
09:19:39     args.n_processes)
09:19:39   File "/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch-03-26-2018/swift/utils/swift_build_support/swift_build_support/shell.py", line 232, in run_parallel
09:19:39     results = pool.map_async(func=fn, iterable=pool_args).get(999999)
09:19:39   File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 567, in get
09:19:39     raise self._value
09:19:39 Exception: ['git', '--git-dir', u'/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch-03-26-2018/swift-integration-tests/.git', '--work-tree', u'/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch-03-26-2018/swift-integration-tests', 'checkout', 'swift-4.2-branch-03-26-2018']
09:19:39 Cloning 'swift-integration-tests'```