Reduction pass idea: remove naming and decoration instructions

From @s-perron in https://github.com/KhronosGroup/SPIRV-Tools/pull/2056#discussion_r235055237:

"Decoration and OpName instruction are considered uses of an id. They could force lots of code to stick around that is not needed to reproduce the problem."

Start by reading the linked pull request discussion and locating the reduction pass that treats OpName and Decoration instructions as uses of an ID. Determine the intended behavior for removing those instructions while preserving the reduced reproduction; the issue provides no named files or tests, so completion criteria need clarification first.

Written by the indexing model from the issue text.