spirv-fuzz: Wrap non-single-entry-single-exit regions in selection contructs

Currently, `TransformationWrapRegionInSelection` only supports single-entry-single-exit regions of blocks. This means that the transformation doesn't support early returns from the function, `OpKill` and `OpUnreachable` instructions etc. Consider removing this requirement.

Start by reading the TransformationWrapRegionInSelection implementation and its existing handling of single-entry-single-exit block regions. Determine how the transformation should support early function returns, OpKill, and OpUnreachable instructions. Done means the single-entry-single-exit requirement is removed while these non-single-entry-single-exit regions are supported.

Written by the indexing model from the issue text.