spirv-fuzz: Add barrier instructions in dead blocks

Barrier instructions must be placed with care, because they lead to undefined behavior if executed under non-uniform control flow.

However, we can place them arbitrarily in dead blocks.

No files, tests, or entry points are named in the issue. Start by locating the spirv-fuzz logic for generating instructions in dead blocks and related barrier tests; done means barrier instructions can be added to dead blocks without introducing invalid non-uniform execution behavior.

Written by the indexing model from the issue text.