Opt: Optimize loads/stores through nested structs

When glslang stops flattening structs with opaques types, we will need to optimize nested structs as we now optimize non-nested structs.

See hlsl.flattenSubset.frag test in glslang.

Original report from @johnkslang

I claim this issue.

Start with the hlsl.flattenSubset.frag test in glslang and inspect how SPIRV-Tools currently optimizes loads and stores for non-nested structs. Determine the expected behavior once nested structs with opaque types are preserved, then verify that the corresponding nested operations are optimized and that the test demonstrates the result.

Written by the indexing model from the issue text.