spirv-val: structure with variable pointer member

`spirv-val` invalidated a fuzzed shader because `OpCompositeExtract` was extracting a variable pointer from a structure. The result id was going to be used in a `OpStore` instruction. So, `spir-val` logged `OpStore Pointer is not a logical pointer.`

Start at the spirv-val validation path for OpCompositeExtract results used by OpStore, using the issue's variable-pointer structure scenario as the reproduction target. Determine whether the validator should accept this pointer flow or report a different error; done means the fuzzed shader is handled with correct validation behavior and a regression test covers it.

Written by the indexing model from the issue text.