SPIR-V: Aggressively prune unreachable merge, continue target

More aggressively prune unreachable code as follows.
When no control flow edges reach a merge block or continue target:
- delete their contents so that:
  - a merge block becomes OpLabel, then OpUnreachable
  - a continue target becomes OpLabel, then an OpBranch back to the
    loop header
- any basic block which is dominated by such a merge block or continue
  target is removed as well.
- decorations targeting the removed instructions are removed.

Enables the SPIR-V builder post-processing step the GLSLANG_WEB case.

StandAlone/StandAlone.cpp

@@ -988,7 +988,7 @@ void CompileAndLinkShaderUnits(std::vector<ShaderCompUnit> compUnits)
 
             // Set base bindings
             shader->setShiftBinding(res, baseBinding[res][compUnit.stage]);
-            
+
             // Set bindings for particular resource sets
             // TODO: use a range based for loop here, when available in all environments.
             for (auto i = baseBindingForSet[res][compUnit.stage].begin();