Fixes#139023.
This PR essentially removes unused global variables:
- Restores the `GlobalDCE` Legacy pass and adds it to the DirectX
backend after the finalize linkage pass
- Converts external global variables with no usage to internal linkage
in the finalize linkage pass
- (so they can be removed by `GlobalDCE`)
- Makes the `dxil-finalize-linkage` pass usable using the new pass
manager flag syntax
- Adds tests to `finalize_linkage.ll` that make sure unused global
variables are removed
- Adds a use for variable `@CBV` in `opaque-value_as_metadata.ll` so it
isn't removed
- Changes the `scalar-data.ll` run command to avoid removing its global
variables
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Co-authored-by: Farzon Lotfi <farzonlotfi@microsoft.com>
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