[SPIR-V] Ensure that cleaning of temporary constants doesn't purge tracked constants (#95303)

This PR fixes a problem in logics of cleaning unused constants, ensuring
that cleaning of temporary constants doesn't purge tracked constants. On
a rare occasion when this happens SPIR-V Backend emits a code that
refers to a non-existent register, earlier related with a constant.
Attached to the PR test case is a minimal reproducer where names of
variables and instructions lead to such a rare coincidence.