Now, version 5.* is all connected to making the uint type, which doesn't quite work.
Generator versions 4 and 6 do not do this.

This reverts commit 2c65069e, reversing
changes made to fa9b465b.

Fixes #1236.

Optimizations used in HLSL legalization now sweep away unused types.

This allow for propagation through structs with dynamically indexed
arrays. This supports recent removal of non-io struct flattening.

This makes struct returns from functions work, but breaks
structs containing arrays, due to limitations in subsequent
transforms in spirv-opt. This is expected to be fixed soon.