glslang -> SPV: 1) Include post switch-break unreachable blocks and 2) Generally, don't emit degenerate (basically empty) unreachable blocks.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@31023 e7fa87d3-cd2b-0410-9028-fcbf551c1848