[llvm-reduce] Remove DIGlobalVariableExpressions from DICompileUnit's globals (#94497)

The 'metadata' delta pass will remove !dbg attachments from globals (which are
DIGlobalVariableExpression nodes). The DIGlobalVariableExpressions don't get
eliminated from the IR however if they are still referenced by the globals
field in DICompileUnit.

Teach the 'di-metadata' pass to try removing global variable operands from
metadata tuples as well as DINodes.