[AMDGPU] Add GlobalDCE before internalization pass

The internalization pass only internalizes global variables
with no users. If the global variable has some dead user,
the internalization pass will not internalize it.

To be able to internalize global variables with dead
users, a global dce pass is needed before the
internalization pass.

This patch adds that.

Reviewed by: Artem Belevich, Matt Arsenault

Differential Revision: https://reviews.llvm.org/D98783