Remove dead Workgroup variables in ADCE.

If there is a shader with a variable in the workgroup storage class that
is stored to, but not loadeds, then we know nothing will read those
loads.  It should be safe to remove them.

This is implemented in ADCE by treating workgroup variables the same
way that private variables are treated.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1550.