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.

SPIRV/GlslangToSpv.cpp

@@ -1630,11 +1630,11 @@ void TGlslangToSpvTraverser::finishSpv()
     for (auto it = iOSet.cbegin(); it != iOSet.cend(); ++it)
         entryPoint->addIdOperand(*it);
 
-#ifndef GLSLANG_WEB
-    // Add capabilities, extensions, remove unneeded decorations, etc., 
+    // Add capabilities, extensions, remove unneeded decorations, etc.,
     // based on the resulting SPIR-V.
+    // Note: WebGPU code generation must have the opportunity to aggressively
+    // prune unreachable merge blocks and continue targets.
     builder.postProcess();
-#endif
 }
 
 // Write the SPV into 'out'.