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.
* Fix#2478. The fix is to just not try to simplify such loops.
* Also added `BasicBlock::MergeBlockId()` and `BasicBlock::ContinueBlockId()`.
* Some minor changes to `structured_loop_to_selection_reduction_opportunity.cpp`.
* Added test.
