This change allows the reducer to merge together blocks even when they
are unreachable, but keeps the restriction of reachability in place
for the optimizer.
Fixes#4302.
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.
Fixes#2120
Enhanced the reducer so that it can merge blocks together, leveraging the functionality extracted from the block_merge pass in the optimizer.
