Prevent OpVariable Id from clashing in Post Process

Tests from https://github.com/KhronosGroup/Vulkan-ValidationLayers/pull/9717 show that when we do post processing we are currently assuming that the OpVariable ID between 2 different shader stages in the same pipeline are different, but there is a small chance they could be the same and produce a false positive

Start with the tests referenced in PR #9717 and trace the post-processing path for OpVariable IDs across two shader stages in one pipeline. Confirm the case where matching IDs produce a false positive, then verify that post processing handles those IDs without a clash or false positive.

Written by the indexing model from the issue text.