Motivated by integrating spirv-reduce into spirv-fuzz, so that an
added function can be made smaller during shrinking, this adds support
in spirv-reduce for asking reduction to be restricted to the
instructions of a single specified function.
Avoids the use of "using" in favour of explicit qualification, to be
consistent with spirv-fuzz. Fixes indentation in a TODO comment.
Addresses and removes two existing TODO comments by moving some helper
functionality into reduction_util.
Related issue: #2184.
