Allow validation to be disabled in spirv-reduce

spirv-reduce runs the validator after each reduction step. There are use cases where this is not desirable, e.g. if one is actually trying to use the reducer to shrink a SPIR-V module that the validator erroneously rejects.

It would be good to have a --no-validation option to disable validation.

Start by locating the spirv-reduce command-line option handling and the code that runs validation after each reduction step. Add the --no-validation behavior there, then verify that reduction proceeds without validator runs when the option is supplied.

Written by the indexing model from the issue text.