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/SpvBuilder.h

@@ -683,14 +683,12 @@ public:
     // based on the type of the base and the chain of dereferences.
     Id accessChainGetInferredType();
 
-    // Add capabilities, extensions, remove unneeded decorations, etc., 
+    // Add capabilities, extensions, remove unneeded decorations, etc.,
     // based on the resulting SPIR-V.
     void postProcess();
 
     // Hook to visit each instruction in a block in a function
     void postProcess(Instruction&);
-    // Hook to visit each instruction in a reachable block in a function.
-    void postProcessReachable(const Instruction&);
     // Hook to visit each non-32-bit sized float/int operation in a block.
     void postProcessType(const Instruction&, spv::Id typeId);