spirv-fuzz: Make facts context-dependent

Currently, every fact holds everywhere in the module. For example, if some pointer is marked with `PointeeValueIsIrrelevant`, you can insert `OpStore`s anywhere as long as domination rules are satisfied.

It might be good to consider context-dependent facts. For example, we could have a fact saying that some variable is not being read from after this point in the function and, thus, we can use `OpStore`s to alter the variable's value. This is essentially an equivalent of `PointeeValueIsIrrelevant` but with the knowledge of the program's semantics.

A benefit of having these facts is that we can alter original variables in the module as well.

Start by reviewing how existing facts such as PointeeValueIsIrrelevant are represented and applied across a SPIR-V module. Define the program points and semantics for a context-dependent fact about later reads before determining how it would enable valid OpStore insertion and affect original variables.

Written by the indexing model from the issue text.